@KeslerGo 

I have had the similar problem before and contacted tplink support, here is the original article:

1． Please go to the WiFi setting of your IOS devices, forget/delete the wireless router’s network(Archer C64), and reconnect to it again.

2． Please set the DNS server to 8.8.8.8 and 1.1.1.1 manually, then go to the WiFi setting of your IOS devices, forget/delete the router’s network, and reconnect to it again.

3． If you try the solutions above and you still cannot solve the problem, please consult Apple's technical support.

TP-Link wireless router will not block any encrypted DNS traffic. When this warning appears on your IOS device, it may be caused by either of the following two reasons.

1.  Wireless Router briefly lost Internet connectivity, resulting in some of the encrypted DNS traffic not being forwarded out. At this point, due to a problem with the judgment of the IOS device, this warning will not go away even if the Internet comes back to the stable. This situation can be solved by solution 1.

2. The DNS server provided by your ISP does not support DoH/DoT, this situation can be solved by solution 2.

Wish it can help you.