DNS issue driving me crazy

Hi, I have a 3CX ready Pi4 on a new subnet. I have assigned a hostname and fixed IP/router/DNS - nothing else. When I set the Google DNS servers it fails to resolve a ping. A windows client on this same subnet can resolve with the same settings no problem. If a the router acts as a DNS server the Pi can resolve OK. The firewall is currently allowing all traffic to the internet. What is different about the Pi?