Self hosted Gitlab: Can't find webpage on standard http port

Hey Gitlab community!

I’ve installed Gitlab-CE on a Raspberry Pi 4 (4GB), running on Ubuntu 20.04 LTS.
I followed the instructions of the recommended installation for this: https://about.gitlab.com/install/?version=ce#ubuntu
I didn’t include a https address at the beginning, instead I’m using:
external_url 'http://gitlab.example.com'

After running reconfiguration and a restart, I still can’t reach the website via the firefox webbrowser.
However I can reach and login on self hosted gitlab if I change the configuration to an alternative port: http://gitlab.example.com:81

I absolutely don’t get it and I’d be glad if anyone had an explanation for this. If more information is required, let me know.
Thank you in advance.