I agree with what you said, however, OP never mentioned pihole being an unraid docker or service, just that the router is pointing to the pihole for dns, which is the correct set up.
There's no reason to bypass pihole for Unraid, the answers should have directed you to start with the logs in pihole to see if that address was being blocked from unraids IP then work from there.
Love Unraid, been using it for a few years at home and has steadily grown from small extra computer parts to a full on overkill server build, love this software!