While installing some software that links into AD it mentioned check domain topology is healthy, I opens site and services and forced replication, however it gave the RPC server is unavailable
The cause was a dns issue on the primary dc, it was set to my routers ip, I also noticed for some reason if I pinged the FQDN of the other dc it gave an external ip, which was strange, I updated dns to loopback and the other dc and sync was able to complete