I think I have had that issue with C&C generals on an older router waay back, Im sure c&c generals uses P2P as well, also another option but I would try the modem first is to go into something like virtual server settings on both your routers and both go into command prompt on your seperate machines, do ipconfig /all
go into your router admin web interface, into virtual server settings or the alike and look for an option DMZ and put the ip address in e.g 192.168.1.253
Your friend should also do the same on his router for example say his ip was 192.168.1.252
he goes to the page with the dmz option, selects it and puts his IP in as well , that way both of you have all your ports open
Sure someone will say thats badly insecure but to me its a great way to initially test that it is possible for your routers to work. Try modems first without routers,next try both on dmz then if that works try port forwarding . The thing is some nat routers the more recently bought ones have an ip stealth type option so you may need to turn that off.
Good luck with it, I remember when I got my first router it was a pain in the butt lol
Keep at it just test one little thing then refine , its a pain I know
