I got it to work for me
I'm running Windows 10, through a KW5262 router, and portforwarded UDP 50000, 1161, 9054, 8054, used /gips and /ips so I can DMZ my local 192.168.1.x IP via gateway router page, tried un/checking upnp in the client(typing /upnp doesn't work for me as it keeps failing to open ports) and I still get dropped ingame after 30 seconds. /porttest in the cncnet channel chat tells me all my ports are open after all that work, but in the game lobby, it tells me they're all closed.
I found out that I could only play the first game I join after freshly opening cncnet, but drop on all other subsequent games. The RA client also takes forever to open for subsequent game sessions as well. Already "allowed" upon firewall prompt, and Windows firewall automatically adjusts to port changes through the router, and finally tried disabling it so it doesn't have to do with the firewall.
The only solution that worked for me was to sign out of Windows and sign back in each time I wanted to join/host a new game lol
Also, using /porttest in the game lobby causes me to drop for that game session.
RA is a pretty old game, so it's a miracle it even loads on win10. I guess it has to do with your client being out of sync, lagging with the previous game session, the game process tree not being completely killed, or just some serious compatibility issue. I don't get this problem at my university which runs Win7, never drops and is always smooth using fresh install and default settings.