This old 2015 thread says there was not (at that time) yet support for same-network (LAN) users to play co-op.
I'm having the same issue; has support for this been implemented yet?
(Friend can connect to my co-op game but error establishing connection when trying to Play the mission.) Both latest RA3 v1.12. (If it's relevant, I have the non-Steam version, friend running Steam version using hooking in C&C Online.)
PS: As per the common issues thread, we did add C&C Online firewall rules, but didn't change Options.ini because the FirewallPortOverride setting is 0 there (which I presume equates to 'unset'), not 16000. (It would seem logical that, for LAN-based play, C&C Online has to be able to connect each of us on different ports since we're going via the same public IP address. I assume unsetting this value is what allows for this if it is supposed to work.)
Edited by ramminrigsby, 06 April 2019 - 05:46 PM.