1.1.1.1 does not resolve but 8.8.8.8 does

Hello, strange occurrences on my office network - when I have 1.1.1.1 in my router’s dns setting and 1.1.1.1 is is being used by dhcp which distributes that ip number to my computer I can’t resolve one particular domain on an external server I control. Other domains on the same server are accessible. That servber has one ip address in use by all domains.
If i switch the router’s dns dhcp setting to 8.8.8.8 all returns to normal.
I’m stumped on why one particular website does not load. Also, when this happens I can’t ping that server’s ip address, BUT I can access other websites on that server.

Anybody can suggest some troubleshooting I can try?

FYI when doing a tcp dump for port 53 I see servfail when using 1.1.1.1 with one specific domain. Does that mean servfail on the server hosting the site or cloudflare server fail?

SOLVED:

whois 1.1.1.1 | grep country

shows 1.1.1.1 is an Australian-registered company. If using ConfigServer firewall (CSF) and filtering by country - which many admins do - with either CC_ALLOW_FILTER or CC_DENY then dns lookups won’t work. Thus no access to site.

Hello Cloudflare please make this a footnote somewhere!