For around two years now, I've owned a T-Mobile Sony Xperia Z C6606/C6616 and I haven't had any major problems with it. It has always been running Android JellyBean and according to the update center, this has always been the most recent version for my T-Mobile branded Xperia Z.
Two days ago I wanted to try something new, so using this guide, I flashed my phone to Lollipop 5.1.1 and it's been running fine so far.
However, my cell reception and mobile data have significantly gotten worse. My texts rarely send and my calls often drop. My wi-fi is fine but when I only turn on mobile data, the internet loads slowly or not at all.
I have restarted my phone multiple times since flashing but nothing has changed, and turning airplane mode on and off does nothing as well.
I know that it isn't because of my location within the T-Mobile coverage because my sibling, who I live with and is also using T-Mobile (but hasn't flashed), gets great signal. (He has an LG G4 running Marshmallow.)
I am currently using the "default" APN settings that came with the phone.
Has anyone else experienced the same problem, or knows of a possible cause?