I've got an HTC One M8, on the T-Mobile Network. I recently unlocked and rooted my phone. I downloaded the Paranoid Android beta ROM and installed it along with gapps. All went smoothly and booted up fine. However, I noticed my phone wasn't connecting to LTE. All I could do was make phone calls. I tried flashing the ROM again, same issue. I then tried the Cyanogenmod ROM and also had the same issue, so I finally restored my phone's nandroid backup so I'm all good again.
But I'm curious what the issue might be. I also tried taking out and putting back in my SIM card etc. Is there something I'm missing that I need to do?