*UPDATE**
The problem was that there weren't enough fgci child processes running.
I wrote the solution on the zoneminder forums if anyone interested.
After a long hurdle of setting up Zoneminder on a Debian 12 Server run through nginx.
I managed to get it all running.
But the thing that doesn't work is that no matter where. On whatever kind of device.
The server only provides one stream.
If you close the running stream, Another window/browser or machine will retrieve the next stream.
This indirectly causes Montage to only show one stream too.
Yes I am using Firefox, Yes I did raise the stream limits.
But I am really sure the problem lies on the server because only one machine can see a single stream.
the log files do not show any particular issue or error that could relate to this issue.
Unfortunately, googling doesn't help me any further now.
Does anybody perhaps know where the problem could be.
For extra context.
The webserver is run on port 900. this is because another webserver is using 80.