Hello,
Is the issue still relevant?
I cannot advice you on configuring Mullvad as I have not done that, however, it might be helpful if you could provide some topology graph of your intended network to better understand your needs. Also, I would like you sent a troubleshoot file to view your current configuration and log entries for better understanding.
To generate the troubleshoot file, access router's WebUI, go to System -> Administration > Troubleshoot section and download troubleshoot file from there.
Best regards,