My iphone 6 has been bent and bent back twice. Cellular service worked fine untill recent smash of the phone (throw it to the wall hardly), and the two volumn button area bent again. After I opened the case, and bent the area back, the cellular service failed. it continueously displaying searching for network.
Here is a few things I have tried, which sometime works, and not work.
1. contact ATT to reset the network login from their side. [looks like the most useless one. sim chip are very standard, and robust, shall not be the problem].
2. wipe out the phone and restore it from back up. Remember to back up your files on your computer before wipe out it. [worked for the first two times, last for one day. then back to searching again.]
3. Tighten the vibration module screws. In one attemp to bend the phone, I notice the right screw to tighten the vibration module on the phone is falling out, and I screwed it back. Restore the phone, and it works out nicely for a day before it failed out again.
4. Tighten all the screws on the two volumn button area, and top left area. The cellular service comes back for two days. before it start loss it again.
5. Always try to Push home and power button at the same time to restart the phone, while your phone displaying searching for cellular network. It is a hard reboot, and will reset some software setting for network searching.
In my experience of my phone, it looks like the bend issue of the phone will cause some parts to disconnect internally. Most likely is the two cellular antennas: one on the top left, and one at the bottom right. Also, if you use the phone vibration a lot, then after a some time, the screw tighten the vibration module is lose. This will cause the bottom right antenna disconnection as well.
while your phone have cellular service, type *3001#12345#* in the phone call dial panel. it will direct the phone to field test mode. You will see how your phone behaves while connecting to cellular network.