I just imaged ADSBx for Raspberry Pi 4B. I want to access the settings webpage remotely but I have no router access for port forwarding. I use a UPnP utility on my other systems to forward ports, but I am unsure how to use terminal on ADSBx. Can I SSH and just do a normal Linux install? Or is there a GUI / Desktop environment?
There is SSH access, you can find info here. Build Your Own ADS-B Receiver - ADS-B Exchange
That said I’m not sure why you would need access to the configuration page remotely, once you set your feeder up there is really no need to go to that page again.
For remote management of a feeder (e.g. to adjust gain settings, apply updates, etc), you might want to look at tailscale. The ADSBx 9.x image (currently beta) has it preinstalled (intended for remote assistance from ADSBx staff). I’ve not tested this thoroughly, but you should be able to (when logged in locally with ssh) just do sudo tailscale login and go through the usual setup to join/create your own tailnet.