Server is setup correct and running and when I connect using the invite code it prompts for a password. Upon entering the correct one looks like it's loading for about 30 seconds before crashing to desktop.
I restarted the server using the Nitrado web control panel and noticed it said something about 'updating' at the top. I checked server and client versions and they match.
Behaviour has now changed to crashing back to the main menu of the game, but I still cannot connect.
I was advised to ping @winged condor by Benjamin Bridgeman.