Wifi want take

Start here on this guide for troubleshooting network connection errors: http://support.xbox.com/en-US/xbox-one/networking/network-connection-error-solution

Have you tried taking some devices off the wifi, and then connecting the two Xbox's that don't want to connect via wifi, just to see what happens?

You also may want to take into account if you have multiple wifi connections that both your Xbox and PC are connected to the same one, some modems may have two connections and if they're not on the same wifi it will not work properly.

@taidara: I could, but I'd rather have working wifi, especially if I want to take my system somewhere else where running a cable is not an option.

Motorola SB6121

If you want you can also take your mobile phone and create a wifi hotspot, and connect your xbox to that, it only needs a few kilobytes and second to do the check then go back off