iPhone 6 searching for signal. Wifi and Bluetooth work.

So one day my iPhone 6 would not find a signal. This happened right after an iOS update. No water damage. No physical damage.

Wifi and bluetooth work and I can make calls and send iMessages/sms normally on AT&T WiFi calling. If I am on Wifi that doesn't support AT&T WiFi calling I can send iMessages and access the internet but can not send sms texts or make calls. Everything else works great.

Here's what I know so far:

Any software settings, resets, restores, etc. have no effect on the issue.

For a few weeks I could regain cellular signal by twisting the phone from the top and bottom and/or applying pressure near the top right/center of the device. This no longer has any effect.

I am considering opening the device to see if any cables/connectors have come loose.

What should I be looking for and where is it in the phone?

What parts should I consider replacing?

Any other advice/help?

Thanks!