You really can’t fix it by artificially nerffing because assuming that the platform has random enough dice (which is most likely does for any real needs) the fact that they have thrown poorly before doesn’t mean that they will do so after your changes.

I don’t know how you are keeping track of the rolls but if you aren’t, I would first try that to truly see if it really is the case that the rolls are lower than average. Our memories of things going poorly aren’t objective and tend to enlarge amount of bad outcomes.

Probability is just probability. What separates D&D from a video game is the flexibility of the players and the DM. Lateral thinking wins the day!

My advice? Review your players’ backstories and add in an unexpected ally who shows up just in time to deliver the solution, but at the cost of needing their help with something conveniently related to the main quest. Alternatively, encounter an NPC who, likewise, conveniently assists them out of a sticky spot in exchange for pursuing the main quest.

You could borrow hero points from Pathfinder. Basically, each player gets one or two hero points at the beginning of the session and can spend it to reroll a d20 at any point in the night. You take the rerolled result. Note that these expire at the end of the session and cannot be stacked up for next time.

Use digital dice?

Narrative solution would be to reveal the party has been cursed somehow this entire time. You can then give them a trinket/spell that mitigates low rolls. They get the best of 2 rolls once per combat or something like that.

I have a rule as DM that I will never fudge rolls against the players, but I will fudge rolls in their favor if it fits the narrative. Three players consecutively miss an enemy? Oh no, next turn it got a critical fail and then failed a Dex save, slipping and landing prone. I guess the players get advantage on melee attacks! Don’t do this often, but I’m the right spot it’s fine.

Monsters can just be dumb, too. INT of 6 means those Blobby Blobs are gonna fight poorly, attacking the tank and splitting up attacks. Also, the players don’t know your monsters’ stats, so you can make an attack that would get it to 1 HP instead kill an enemy. I also don’t do this often, but sometimes they just get unlucky, and no reason to TPK.

Speaking of TPK, you can let them fail forward. Monsters rarely have reason to attack downed/unconscious PCs, so let them roll death saves, but it usually takes a while to die. Everyone’s unconscious? Stop combat, no more death saves. Instead, they wake up as prisoners and have to escape. I had a particularly fun one where my players would have died to a trio of night hags (if you’ve played Curse of Strahd, you know the ones). Instead, they woke up and were given a mission, but the bags also stole parts of their “souls,” taking Max HP from one player, speed from another, giving one a trait that they would lose all hope, stealing another’s eye (disadvantage on Perception), etc. Then they had to fulfill the mission, but later came back at a higher level and beat the hags, gaining their souls back. Everyone loved getting revenge!

Tell them to roll real dice for attacks and trust them to not lie.

They need to roll the d20 and not the d12.

But seriously, the party can figure out AC, but not hit points. If something is dragging on, just have things die on the next hit. I know some people would complain about it, but it is the group’s game, if something isn’t fun, change it. You have the power.

Also, some things fight to the death, but it is also ok to have some things surrender or run away. Fighting a group of bandits? When one goes down, have someone scream “Bear! No!” and run away crying. Killed half the goblins? The rest turn tail and run.

There are many levers to pull, and don’t be afraid to use them.

I’m partial to semi-narrative combat. My baddies rarely fight to the death, so they’ll disengage and run away when they take too much damage or we start to get bored. Could you try something like that?

Nerfing the CR doesn’t seem terrible, so long as you have a backup plan in case they roll well.

Another option might be using Inspiration or another meta-currency to allow the PCs to push a near miss up to a hit. I’m playing Cyberpunk RED right now, and it has Luck, which kinda does that.

Alternatively, you could try to design fights with non-murder win conditions. Like the PCs are just trying to get across a bridge, etc.

Can you have everyone roll a 50d100?

I’m curious if there’s a technical issue at play that’s influencing the random number generator. Such as a script blocker or privacy tool. There should be no discernible pattern between the players.

There’s also the phenomenon that true random doesn’t feel random. Such as when a music player feels like it’s shuffling between the same 20 songs instead of the whole list.

Players not succeeding on rolls isn’t necessarily a bad thing. It can increase tension in your encounters and is ultimately something you can’t control directly. You can fudge dice on your end, but honestly this reduces tension and if you’re not decisive about it can be pretty noticable.

So what can you affect? First, the stats of the monsters you’re throwing against them or the DCs that you’re looking for can be adjusted. If they’re consistently struggling more than is enjoyable, you can always scale things back a bit when you’re prepping encounters. Inspiration can also be a great way to let the players swing things in their favor without feeling like they’re being handed a success. There are also times when you can throw in a “oh actually, roll that with advantage” with some reasonable excuse, which also doesn’t feel like a gimme. But aside from this you have a whole toolkit in your ability to shape the world around them.

Magic items can be really helpful here. A staff with silvery barbs or bless or something similar can be a great way to put some die manipulation into your players’ hands, especially if the party composition lacks things like this. But don’t forget that you can also use set pieces.

Maybe the ceiling above an enemy has a weak point, or they’re standing directly under a chandelier or some nets. Maybe an enemy prefers their own safety in a crowded fight over that of their allies and drops an AOE that brings some friendly fire. Maybe some magic item or wards on the monster are responsible for part of its AC and dispelling it might help. The options are only limited to what you can imagine.

For great tension in a game, though, you want there to be risk of failure. Resist the urge to just hand the win to your players. Your job isn’t to make sure they win every encounter, that’s their job. They’ll sort it out themselves. Maybe they don’t, though, and they’re taken alive. Maybe the entire party wipes and properly dies, but in 100 years someone enters their tomb and ressurrects them to perform some task that they’re uniquely suited for. Maybe some sketchy wizard who wants something from them and has been scrying on them for the past week teleports them away, only to back them into a corner and try to make them do something against their interests or beliefs. Maybe they’re captured and lose this particular fight, but are able to rest wherever they’re being held and find others to help them secure their belongings and fight back.

In general, don’t worry about how the dice are rolling. They’re meant to be out of your control, but you can do literally anything with the world.

The luck system might be something to look into. It can help mitigate consecutive failures while maintaining balance.

Why would a fix be required? It seems like you expect the bad rolls to continue.

Go for a tpk.

Then, once they die, have them wake up with slave bonds and have to complete 3 trials for their subdemon overlord in order to be freed.

During that time, get a DM screen and hide your rolls from them so you can fudge a little to make the game more fun.

You could try attacking with hordes of low CR enemies instead of a few high CR ones.

My recent game has been similar. Not because I roll badly, but because the GM is incapable of rolling below a 16.

And I say: if they die, they die. You’re playing digital, you’re not going to run out of character sheets.