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.

Give them custom weapons or spells that are most effective when you roll low.

You might consider adding a puzzle element to encounters that can lower the effective difficulty with clever maneuvers or strategies.

As an example, fighting in a room with a big chandelier overhead. A player can cut the chandelier at the opportune moment to pin a major adversary, allowing them to coup de grace or simply flaunt their victory to the villain's face.

Or perhaps fighting in a room full of mirrors that allow a clever player to reflect a gaze attack. Or doing Battleship style combat, where you have to pick the square of a hidden enemy, but you guarantee a hit if you guess correctly.

In general, try introducing non-dice resolutions to the scene - guessing a magic word that disables a key piece of enemy tech, baiting enemies into an area or formation before springing a trap, completing a ritual that can summon a powerful ally by solving a rubric cube.

If all else fails, you can drop some nice loot them. Awand of fireballs or Staff of the Sun or similar high powered magic item, for instance. Doesn't matter how you roll with these, you're going to have some fun.

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.

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