Sometime error when converging my WiFi

The Fairphone sometimes cannot connect to my private network. However, while other Android phones can do. I can then only connect via the guest network. The Fairphone and my router have the latest firmware installed. A reboot does not solve the problem.
Sometime I need to switch then the WiFi channel on my router from auto to a dedicated channel. Some time this help temporary. But now this would not help.
What else can I do?