As reported back in 2023 (https://community.tp-link.com/en/home/forum/topic/604494) the app doesn't allow IPv6 DNS server IPs in the ULA (fd00::/8) range to be used, saying the IP is invalid.

This isn't correct, they are just as valid as RFC1918 IP ranges, such as 192.168.0.0/16, they just aren't globally routable, the same as RFC1918 addresses aren't.

This would solve the problem of when, not if, you change ISPs or if your current ISP changes the subnet allocated to your connection for whatever reason.

For more info, I posted details to this thread along with other weird IPv6 issues and quirks I've come across since upgrading to my Deco BE65 Pro routers:

https://community.tp-link.com/en/home/forum/topic/838180