I came up with a solution that worked for me and my friend. We both opened all the necessary ports, turned off the router firewall and also our anti-virus programs. It was when we disabled the anti-virus that we could finally connect to each other.
Hope this helps someone
EDIT:
Thought I could provide the ports I've opened if someone is wondering which ones they need to forward.
UDP 3659 - 3659
UDP 9999 - 9999
TCP 80 - 80
UDP 80 - 80
TCP 443 - 443
UDP 443 - 443
TCP 1024 - 1024
UDP 1024 - 1024
TCP 9960 - 9960
UDP 9960 - 9960
TCP 18000 - 18000
UDP 18000 - 18000
TCP 18120 - 18120
UDP 18120 - 18120
TCP 18060 - 18060
UDP 18060 - 18060
TCP 27900 - 27900
UDP 27900 - 27900
TCP 28910 - 28910
UDP 28910 - 28910
TCP 29900 - 29900
UDP 29900 - 29900
TCP 20000 - 20100
TCP 22990 - 22990
TCP 17502 - 17502
UDP 17502 - 17502
TCP 42127 - 42127
UDP 42127 - 42127
TCP 14000 - 14016
UDP 14000 - 14016
UDP 22990 - 23006
UDP 20000 - 20100
TCP 25200 - 25200
UDP 25200 - 25200
