iPhone 6 is constantly searching for signal

I am currently on the 10 beta software. The problem started after updating to 9.3.3. My service works for about 24 hours anytime a new software update comes out. Then I am able to turn airplane mode on and off for a few time to get the service to work. This fix eventually is unable to work and my phone shows "searching..." status. I have tried turning my phone on and off, restoring, taking my sim card out, replacing my sim card, and resetting my network setting and none of those have provided more than a temporary fix. I have also taken my phone in to the Apple and AT&T store, but they were both unhelpful. Apple offered to replace my phone for $300 since it was out of warranty. They were convinced the problem was related to the hardware. If that was true I would think the service would not be working at all instead of temporarily. I would, however, like to know what component I could replace to create a permanent fix if it is actually related to hardware. Thanks in advance.