usually there should be no compatibility issue if both routers set up correctly

both 2.4Ghz and 5ghz wireless network disconnect? were both wifi leds stay on or flashing? do you have any laptops that are using killer wireless card? I've heard some cases about that killer...

I see that the router can work as a PPTP VPN/IpenVPN server. http://www.tp-link.com/en/faq-1751.html it also supports IPSec VPN

that's really a long time...does it have something to do with the phone line too? ignore it if you test with other modem. what would be the light status on the modem? is the DSL light still solid on?...

change the location of the extender maybe. and your phones and pads are too far away from the router, consider the powerline extender if you have one power circuit in your house

did you reboot the router? try it if not. otherwise, set it from 10.am to 11am to test whether access can be regained.