wireless backhaul under AP mode is already possible, read this FAQ: http://www.tp-link.com/en/faq-1842.html

Good luck!!!

Devices connected to the Deco network no matter via WiFi or Ethernet, should be able to talk to each other. If your wired devices aren't able to talk to wireless devices, check their IP addresses,...

What VPN exactly your son is using mind if I ask? very interested to tested it out from my side.

I too think a reset and reconfiguration would make things right, good luck. If not, contact TP-Link, they are always responsive.

Actually, Guest network still works with Ethernet backhaul, only coverage of Guest network is somehow shorter compare to primary network.