Try reinstalling qbittorrent
sudo apt-get install --reinstall qbittottent
In Transmission >Preferences > Network
Port used for incoming connections: 6882
Pick a random port every time Transmission is started = UNticked
Use UPnP or NAT-PMP port forwarding from my router = UNticked
Now if you've forwarded port 6882 to the PC's IP address in the router .. it should all work
You can also check the port is OPEN by clicking the "Test Port" button in Transmission.