ISA Declaration isn’t available on the app at least the android one anyway.
For any isa related queries, you can email trading212’s ISA team directly, they are extremely helpful.
From what I remember though, the declaration pops up on logging in.
If you don’t see it for some reason, I think you can when logging in on a desktop.
(Not 100% sure though, hence the email above).
Yeah got it sorted on desktop