Does this occur when you only access the Apollo from outside your local network, or does it happen from your local IP as well? If it only happens outside it could be a bug, as I have not tested this case.
If its happening in both cases then sounds like the SD card got corrupted and is not accessing your already configured flag.
I am able to reach the dashboard normally if using the LAN address, however when using the WAN address I am prompted for Initial Setup regardless of whether I am physically connecting via LAN side or WAN side. I've re-imaged the SD card (the checksums match, written with balena Etcher, no other settings changed), and the problem remains. The port redirect is simply WAN_IP:81 -> LAN_IP:80; confirmed in multiple browsers, cookies, etcetera.
Using Chrome DevTools, I was able to enter my password in the form (behind the Initial Setup modal), and submit. The password input is replaced with the "Loading..." indicator, but never goes beyond that. I get XMLHttpRequest timeouts in the console.