Additional to router update, router restart and disable 5 GHz, you could try out to change security settings in your router (“WPA/WPA2 (very high secure)” instead of “WPA2 (high secure)”) (for testing).
On the FP2 (Android 7) you could try to change some settings:
- Location - Mode - “High accuracy” (GPS only)
- Location - Mode - (3 dots upper right side) “Scanning”: disable Wi-Fi and Bluetooth scanning
The problem which you describe is happening also for many other people with some WiFi networks:
https://bugtracker.fairphone.com/project/fairphone-android-7/issue/139
I personally will wait for the Android 9 upgrade and test there again:
For time being as a workaround, I added another router/repeater/access-point device to my network. I did it with “TP-Link-TL-WA901ND” and I didn’t have a WiFi disconnect since 3 months so far.