@starmanj 

Could be something with the network settings or the auto join function may be disabled on the phone.

I fixed it by rebooting all M5's. Tech support said you need to reboot every time you enable/disable fast roaming! Why doesn't the software do this automatically then?

Tech support also claims that fast roaming won't work in AP mode, you MUST have a deco in router mode. What the??? Why don't they say that in the settings then? Come on TP-Link!