Cannot connect to gitlab web interface


My git pushes (using SSH) to my gitlab instance are working fine, however I cannot connect to the web interface, no matter what browser. For example Firefox gives me the standard message:

Unable to connect

Firefox can’t establish a connection to the server at XXXX.

The site could be temporarily unavailable or too busy. Try again in a few moments.
If you are unable to load any pages, check your computer’s network connection.
If your computer or network is protected by a firewall or proxy, make sure that Firefox is permitted to access the Web.

I am running gitlab on CentOS Linux release 7.6.1810 (Core)

System information
Proxy:		no
Current User:	git
Using RVM:	no
Ruby Version:	2.4.5p335
Gem Version:	2.7.6
Bundler Version:1.16.6
Rake Version:	12.3.1
Redis Version:	3.2.12
Git Version:	2.18.1
Sidekiq Version:5.2.1
Go Version:	unknown

GitLab information
Version:	11.5.5-ee
Revision:	e515ce9
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	postgresql
DB Version:	9.6.8
SSH Clone URL:
Elasticsearch:	no
Geo:		no
Using LDAP:	no
Using Omniauth:	yes
Omniauth Providers: 

GitLab Shell
Version:	8.4.1
Repository storage paths:
- default: 	/var/opt/gitlab/git-data/repositories
Hooks:		/opt/gitlab/embedded/service/gitlab-shell/hooks
Git:		/opt/gitlab/embedded/bin/git