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).