Was having this issues for months. Searching hours on google to find an answer. I replaced the Lightning Charging Assembly which included a cellular antenna, cleaned all the grounding points in the case, and replaced the antenna flex ribbon. None of this worked.
1) Software wasn't the issue. DFU restored, everything, it still continued.
2)Cellular settings being changed had no effect on the signal.
3) Hardware was the obvious in this case because "smacking" the phone caused it to regain signal.
A few days later I noticed my phone said "NO SIM" and started to think. This is what the problem is. The sim slot in the iPhone 6 can get deformed by slight bending in the phone. This provides the SIM card with a hairlike gap to wiggle around in. The simple fix for this is to take a sheet of printer paper and cut it the exact size of your nano-SIM card and place it on the plastic side of your SIM card which faces up and then insert it. Do this carefully and make sure the paper doesn't crinkle/fold in half. This tightens the SIM card very slightly but just enough to keep it tight to the gold connectors.
My phone was loosing signal, and saying "Searching" for weeks, and after this it is working 100%!
Three days and strong signal ever since.
Goodluck!