I have my private DNS value configured for 1dot1dot1dot1.cloudflare-dns.com and the Samsung browser does not appear to acknowledge the use of DNS over TLS when tested using the cloudflare site https://1.1.1.1/help.
Does Samsung browser acknowledge the configuration of DNS over TLS?
Well you are right that this issue shouldn't be caused by your ISP. But you are incorrect that S Internet does not work with DoT,as evidenced by this screenshot
S Internet is Samsung Internet... It's an abbreviation instead of always saying Samsung this, Samsung that... especially since all those apps used to be just called S this, S That (S Note, S Health...)