The router doesn't have file shares, so it doesn't show up on file manager. You most likely need to connect to that with a browser by IP-address. But as port forwarding can have severe security issues I really suggest that you learn more on what you're trying to do and understand the implications before poking holes to your firewall.

I'm not comfortable to provide step-by-step instructions since doing that wrong can cause all kinds of havoc on your network (and the whole apartment if you happen to have IoT-things around).

One solution would be sending email requesting for them to open ports for you, you can tell them that you need open ports for security cameras and use that port for reverse proxy. That way you dont need more than one open port. Second option is cloudflare tunnels, they do not require open ports at all, but they do require for you to buy domain though. Last option would be to use VPN like tailscale which also do not require open ports at all. All you need is clients installed on devices and connect them on the same network.

Hope some of these help, i actually use all methods for different situations and each of them have pros and cons, but those are all reliable methods.

not helping with the opening of ports or providing the admin username/password for my router

Oh, so it's not your router, it's theirs. Just get your own, then you can open ports and let anyone on the Internet into your network.

What are you attempting to do here? Host for any generic user? Host just for yourself? From only one remote device? From many remote devices?