I tried removing the volume buttons/mute/upper microphone part considering it was the last modification since the phone was working and it was to no avail. Tried messing around with the antennas, removing one and the other, etc, but the only thing I learned was that neither the top or bottom antenna is used for Mobile Data.
So I picked up yet another phone I had lying around. Switched boards entirely. I gave the lower antenna the benefit of the doubt in that the first replacement might be damaged as well considering the phone was water damaged - but nah - it's the same crap. I put the board in an entirely new case and the problem isn't solved. Considering there's nothing possibly replaceable to fix the issue, I tried to inspect the area around where the lower antenna connects. Compared the two boards that I have, shining a light on it and using a magnifier and all, thinking maybe I could've accidentally scraped it a bit somehow, but guess what - there isn't a nanometer of a difference between the functioning one and the malfunctioning one. It's fascinating.
I can only conclude a spell has been cast on this cursed phone and that it is in fact irreparable by any humanely possible means.
Amid all this, I'm some 10 hours down on my lifetime and not a cent richer. Maybe this is why the blokes who fix this stuff charge so much money for any repairs...
Cheers.