suggested solutions for signal problems
I had severe signal problems twice with my Fairphone 2.
App blocking signal
The first time the signal dropped when beeing indoors, still having the operator sign but I was not able to make or receive calls, nor connecting to wifi. The first suggestion was a broken contact in the hardware or the antenna. If I remember correctly (I had some chances of troubleshooting with my phone) I went to the secure mode and checked if the signal and connection to the operator worked. As it did I started deleting several apps I had installed, finding out that one app blocked the signal.
Loose Antenna:
The second time last week, the contact to the operator broke completely indoors (no operator available) or I I had low signal, changing from 4G to 2G, no signal, no operator connection. while making calls the connection broke down from one second to the other.
The phone fell on one corner (half meter hight) two days before the problem occurred, the day before I made a software update.
I went to the operator to get a new SIM which didn’t solve the problem. I checked the forum, which didn’t help me that much. I tried to contact Fairphone for repair suggestions, I set the phone back to factory settings, checked it in the save mode if any app causes the problem. neither of them helped.
I fond one post helping me. It described how to get to the antenna, which may can get loose.
When you remove the display the the Antenna can be seen on the back on the left side. Left to the microphone module the antenna is clipped on a small plastic pice covered under a small plate which you can take out (you need a small torx-wrench. The antenna can be clipped back to the piece, then you should have full signal again.
(notice, the antenna can get loos because you dropped the phone like me, or due to other reasons. it is a simple clip-connection.)
hope that may be some help.