It’s going to be up to your DM for this one. It would be valid to rule that since you are changing the “number rolled” then that could mean that “the d20 roll for an attack is a 20”. It would be equally valid to rule that since you are “ignore[ing] the die roll” then there is no d20 roll.
Personally, I’d rule with the later. I think the language “you ignore the die roll” means that you are no longer rolling “the d20” as referred to in the rules.