I recently setup multi port, and put some configs here to save myself and hopefully others from having to type
in all 50 30000 based port numbers for large camera setups.
https://wiki.zoneminder.com/Multi_Port
The guide to follow is on medium and linked on that page, but here's a link as well:
https://medium.com/zmninja/multi-port-s ... 836a336c93
In my experience, I set it up for https first, but it didn't quite work right, so I changed it to http for the time being. I didn't bother troubleshooting, as it was only for a LAN. I set it up on Debian/Devuan Buster/Beowulf. I didn't have any issues.
Ports.conf, the 000-default.conf, then the zoneminder options, and restart apache. You should see apache listening on all the ports with netstat or ss as mentioned. Remember that you made these changes, when troubleshooting later. Otherwise, you can end up with stream errors, and not realize it's because you left some part of this configured.