It can be router related. In the past I had an old and quite bad router which used to drop connections often for certain WiFi cards (not all) in WPA2 mode. I replaced it with recent D-Link DIR-825 and those issues were gone. If your issue occurs with any router, then it's likely Jolla related.

If you can, try also installing alternative OS on your router (such as OpenWRT). That can sometimes help to solve these issues, since they can be software related. Be careful with this though, it can brick it or otherwise make it impossible to easily revert. Research this thoroughly first.