What you are saying is partially true.
If I left Guacamole on the same IP as UNRaid, that would be true, but I moved Guacamole to a unique IP, different than UNRaid or any other container. I have other containers running on their own defined (static) IP, on port 80, without issue.
i.e. (these are not my specific IP's)
UNRaid is running on 192.168.1.10:80
ContainerA is running on 192.168.1.11:80
ContainerB is running on 192.168.1.12:80
But when I configure Guacamole to run on 192.168.1.13:80 it does not load on 192.168.1.13 in my browser, but it does load on 192.168.1.13:8080.