Internetworker wrote UPnP is a bad idea and is always vulnerable for an attack. if the default being enabled is true then it's a serious matter and sad. yeah, agree....any tplink rep care to comment...

Sorry, don't know about the UPC router...seems like it's issuing the lan address. You could try and setup deco in AP mode. You may want to delete your external IP address from the post!

Seems like upnp is always enabled for deco m5 & m9, and you can't disable it. maybe something else is blocking it? inbuilt antivirus/intrusion prevention? Or maybe plex needs port forwarding?

One suggestion, set up the guest network and disable 2.4ghz band (or 5ghz band). then connect the apple devices to the guest network and see if that narrows down the connect/reconnect problems. I...

Did you try a factory reset of the TV? (not just a turn off/on).

Yes, strange I agree. Just to update, as few days ago i swapped the culprit google hub back to the main ssid and turned off guest wifi. No issues at all since then. Originally I used the same SSID...