It's clearly a hit and miss when it comes to fixing this problem as I have ran into it before and tried various attempts with no luck.
For an iPhone 6 this time I tried:
1. Swapping out the sim
2. Resetting network settings
3. Checking for updates (all is updated)
4. Swapping the charger port
5. Swapped out the screen assembly
6. Reconnecting the antenna on the top left
7. The infamous drop attempt several times
8. Restarting multiple times several different ways
9. I swapped out the entire logic board over to another iPhone
''''''All with zero luck''''''