How do I properly configure the various external urls?

I’m a bit confused about how to properly configure external_url , registry_external_url , pages_external_url , and mattermost_external_url .
I have installed gitlab-ce on a centos 7 box SERVER , and set-up port-forwarding on ports 80, 443, and 2222 (my isp filters 22) so that my co-workers working remotely can access it. firewalld is also configured to allow communication on those ports.
I use a dynamic DNS service ( No-IP ) to track the external IP of my router, No-IP s free tier allows up to 3 hostnames, so I have DOMAIN , gitlab-DOMAIN , and mattermost-DOMAIN all pointing to my router’s external IP address.
setting the following in /etc/gitlab/gitlab.rb :

external_url gitlab-DOMAIN
mattermost_external_url mattermost-DOMAIN

results in https://gitlab-DOMAIN working, and https://mattermost-DOMAIN failing with ERR_CONNECTION_TIMED_OUT .

How do I fix my URLs to get mattermost working?
Can I get registry , and pages working as well without needing additional hostnames from No-IP?