I have been using Cloudflare 184.108.40.206 DNS for years without any problems but recently discovered that Samsung is blocking users who use it (and possibly other custom DNS services) from accessing the Galaxy Store App. Disabling it is the only way to restore access until Samsung restores access to those of us who use these privacy focused services.
By the sounds of it, it's probably dns over tls, which is still fairly new. Dns over https uses a single port, and businesses still use it.
It would appear to be the case. I found an app to force DoH (DNS over HTTPS) called DoHGuard and selected Cloudflare and it allowed Galaxy Store App to connect.