DNS changes not effect. already try cloudflare and google dns.
After some some tests. I found that when I switch to 4G then tapo works fine. not work when switching to 5G SA only.
I use jio (India) sim. and jio 5G is ipv6 only internet..
But, if I use the same sim with a 115USD cheap ZTE MC801A device, then tapo works fine on both 4G and 5G SA only. but sadly it does not work with a 450 USD device. 
some information related jio 5G ipv6 here : Jio 5G - IPv6 only on transport - anuragbhatia.com