Unfortunately we do not know why the WiFi drops off in some cases. We were never able to reproduce the issue on our own test network. I don’t think it has to do with how far the router is to your controller, rather, it has to do with the router kicking the device off its network for some reason.
We did release a new version of the firmware 2.1.9(9) just today, which contains a new feature that allows using a program to trigger a reboot on a regular basis. So if you have WiFi reliability issues, you may want to give this a try to see if rebooting once per day or maybe per 2 days etc can help.