Can only connect to Octoprint on extern port 5000

What is the problem?

I have an octoprint server running behind a standard router firewall (using TP-Link Deco routers). I can get access to it if I forward external port 5000 to internal port 80. However, if I use any other external port (e.g. 5200, 8000, etc.) it does not work. The reason I'm trying an alternate external port is that I actually have 2 octoprint servers (running on 2 different RPI's) and I'd like to use different extern ports to determine which one to connect to.
My understanding was that with HAProxy, it listens on port 80 and then forwards it to internal port 5000. However, why should it matter what external port I use? If I correctly understand how the router works, it just takes any external request to port 5000 and forwards it to the specified internal IP at port 80. Why should it or Octoprint care what extern port I use as long as it gets forward to port 80?

What did you already try to solve it?

Tried various external ports, but only port 5000 works.

Have you tried running in safe mode?

No relevant

Did running in safe mode solve the problem?

Not relevant

Systeminfo Bundle

You can download this in OctoPrint's System Information dialog... no bundle, no support, unless the reason you couldn't retrieve the bundle is your network issues
octoprint-systeminfo-20260203125846.zip (14.0 KB)

Additional information about your setup

2 Octoprint servers running on 2 separate RPi's