I have the same problem with my Speedport W 925V 
It took me a while to get a connection at all. This was only possible if I switch router wifi security settings from “WPA2 (very high)” to “WPA/WPA2 (high)” as described here: https://bugtracker.fairphone.com/project/fairphone-android-7/issue/276 or Fairphone 2 cannot connect with Wifi with WPA2
Deactivation of 5G Network was helping a bit, but still I’m facing disconnects from the router every 5 Minutes(!)
I added my log output here:
https://bugtracker.fairphone.com/project/fairphone-android-7/issue/139