@Eastmaster when you say you got a XE75 for a 3rd location do you mean a third property (i.e a third separate network) or a third location in the same property? How is the network that is causing problems set up; wired/wifi backhaul, ISP router involved, is the M5 or XE75 set up as the main unit, etc? If all the units are forming a single mesh is there a problem when connecting to the M5 units or just the XE75 unit? Lots of variables here to sort out first!

  @Eastmaster if the older devices at the new property are the only ones having issues my first suspicion is that the new network might be set up with WPA3 or mixed WPA2/WPA3 encryption and they don't like that? You could try setting it up as just WPA2 and see if that fixes the problem for the older devices.

Another issue might be that if the networks both have the same SSID and password that the slight differences between the two networks could be causing weird problems, giving the new network a unique name and reconnecting to it could help a bit.

Looks like you've already disabled the mesh technology for the affected devices too so that's a good troubleshooting step for older devices, hopefully you can re-enable that (and the UPNP, SIP ALG, etc) if we solve the actual cause of the problem!

  @Eastmaster 

I'm glad you managed to isolate the issue  It's weird that it caused problems for your older devices but not your newer ones though!