It's a little odd, but while both my brother in London and I, in Florida, can both connect to the same third party online servers, we seem unable to connect directly to each other. We've tried both listen and dedicated servers, on both ends, but while we can see each others servers we cannot connect to each other.
If it makes any difference, my copy came from GoG, and his is from STEAM. I'm not sure if that would have any effect.
If anyone has any ideas, I'd be very grateful for any help.

Have you made sure that that any firewalls on either the computer or your routers are allowing each other to connect? You may need to do some port forwarding for the game.

Make sure you both have the map pacs for the server game you want to connect to. Right click the server for other connect options maybe compare settings. Add to favorites once you find the one that works.
Can you connect to a game on the server yourself . . . can he connect to a game on the server? If not it sounds like a port forwarding problem as Ralackk stated.