The divination wizard’s Portent ability states (emphasis mine)
You can replace any attack roll, saving throw, or ability check made by you or a creature that you can see with one of these foretelling rolls. You must choose to do so before the roll, and you can replace a roll in this way only once per turn.
The PHB (pag 7, "The D20", 3rd paragraph) describes saving throws, attack rolls and ability checks as
[…] the three main kinds of d20 rolls, forming the core of the rules of the game.
All three of them consists of a d20 roll and then modifiers are applied to this die roll. The description of Portent is quite foggy: the first sentence suggests that it allows to replace the final result (d20+mods), while the second part of the description mentions the roll, without specifying if it just refers to the d20 roll or to d20+mods.
Is Portent allowing the wizard to replace the final result or just the d20 roll?
This topic is covered in several answers and questions related to Portent working with other rules/aspects of the game: these report some tweets by Jeremy Crawford. On one hand, the Sage Advice Compendium ("Official Ruling" section) states they are not considered official anymore, on the other hand in the very same section is stated that they could be a preview of rulings that will appear here [i.e. in Sage Advice]. Jeremy’s tweet about this issue have not been still included in any version of Sage Advice Compendium, at the best of my knowledge.