If you have access to other phones that work: Do they use UMTS as well? Or do they switch to LTE or similar?
Maybe the connection of the rf-antenna is bad (popped off the connector). In my case i rarely get HSDPA+ and mostly fall down to 2G until i can replace my rf-cable (which i broke completely -.- ) - although the phone states good connectivity.
Step 11 Picture 3 shows the RF-Cable leading up to the antenna: https://de.ifixit.com/Teardown/Fairphone+2+Teardown/52523
Other than that i would not know where to look. When another FP2 works i would guess that it is indeed a hardware issue like a loose cable and not tied to software.