And why I can't use «na stole»
Well, that would mean they literally are on the table. Which is a fun thought, but that's not what the original sentence says ;)
What's the difference between "u" and "przy"?
przy=~next to, by, close to
u= at somebody's place
there are more meanings but those are most common.
why is this stole and not stolem?
'przy' takes Locative.
I don't know how to insert accents on a phone
Personally I'd recommend using the app SwiftKey, you could use up to 5 languages on the same keyboard.