Same here. iPhone 5 on AT&T. Went through every possible trouble shooting with them, have reset the phone, reset network settings, turned airplane mode on and off, took out SIM card and replaced and restarted and then did a complete restore with iOS 8 again and still have no cellular data. Infuriating. It also got stuck on the apple start screen multiple times while restoring from iCloud backup.
May have to downgrade to 7 until the next beta or until September. Haven't had any issues on my original iPad mini.