Right, so I did some more testing and followed your suggestions. All three Mirrors have the same problem.
The router is configured without MAC whitelisting, login page, etc. Anyone can connect as one normally would in a home network. (user/pass, WPA2 personal). Other devices can connect, from phones, laptops, to raspberry pi’s and obscure chinese webcams, even from other rooms/floors. Changing to an open network also did not help. Using a different brand router also did not work.
The signal level that the mirror display next to my network name is: 47
Connecting to a wifi hotspot on my phone (held close to the device) also did not work.