Opening router ports is pointless for downloading torrents with this image. If you feel you must do this for seeding purposes, then you will have to forward the port at your VPN provider's side of the connection, not in your router. Not all VPN providers offer this feature.
Leave the port 8999 statements as is in the compose file and in the Listening port connection setting in the qbt GUI. I have never found a difference with changing these or even leaving them out of the compose file.
If you are concerned about the Connection Status icon being a red flame instead of a green globe, this seems to be cosmetic only. But you can try changing this setting in the Qbt GUI: Connection | Peer connection protocol to: TCP and uTP.
The "qBittorrent client is not reachable" in red at the screen bottom left means that your VPN connection has died. Your connection to the qbt GUI has also crashed, but you won't see that until the page is reloaded with a still down VPN connection. Restarting the container is the first thing to try.
The vital things to have correct with this image are PUID, PGID, and the host side path of the volumes. There is no room for any guesswork with these.
When having problems reading both the container log and the qbittorrent.log is a good idea.