@simonkapicka, When you removed the old battery, is there any chance your tool(s) slipped and you damaged a flex cable or disconnected a connector inadvertently? Some of those antenna flexes (as most flexes are) very easily damaged, and even the slightest damage can stop functionality.
It sucks to have to do it, but you’ll probably have to open the phone up again and inspect all connections and flex cables. If you can’t see any obvious damage, using high magnification will help. Then I’d disconnect the cables before reseating them, making absolutely sure that they’re fully seated.
Before gluing the phone back together, always power the phone up and check all functionality, as this will save you from having to open it up yet again. (We’ve all done it at least once! It’s just the nature of how these repairs go.)