Meaning … if you would limit the phone to the 2.4 GHz band the connection would be stable?
Settings - Network & Internet - Wi-Fi - Wi-Fi preferences - Advanced - Wi-Fi frequency band
After initial problems I have a stable 5 GHz connection to my 6490 Cable for some time now, so something must be amiss here.
5 GHz channel: fixed to channel 36
5 GHz WiFi standard: 802.11n+ac
802.11n+ac was a problem for me at first, you can set that to 802.11n+a for testing.
Some update on the phone fixed 802.11n+ac for me, though.
Fairphone OS version: A.0110 (January 5 security patch level)