Redirect from 1 IP from selected port to another ip

When I type my public IP of 202.34.56.20:8080 in a web browser, it would redirect me to 192.168.1.1:80.

I think the best you could hope for here is to (possibly) set up a reverse proxy.

In short, you would theoretically take 202.34.56.20:8080, redirect that IP/port combination to an internal server (e.g. 192.168.1.50) on a specific port and then use a web server (e.g. Apache or Nginx) on ex. 192.168.1.50, acting as a third-party, to access the web interface for 192.168.1.1.

Offhand, I am unfamiliar enough with the capabilities of Windows Server 2019 (and specifically IIS), so I cannot say if it could act as a reverse proxy in this instance. But both Apache and Nginx are potentially capable of doing this and are currently available on Windows.

Any proxying would be done in the context of virtual hosts (Apache) or server blocks (Nginx). Note that there could still possibly be unexpected issues when attempting to proxy the router's interface.