@skysurf76 Official response from Ubisoft on their forums...."Hey there, thanks for getting in touch! I'm sorry that you are running into this issue. Due to technological limitations two players are only able to play from the same network if one of them invites the other. Please keep in mind that you will have to complete port forwarding to the system that is the host, although I imagine you've already done so. Here's an FAQ with the ports, just in case.
Alternatively you both can play if you get invited to groups by people outside of the local network. "
Their explanation doesn't make any sense. They took their xboxes to their dads and it magically worked fine, unless the kids did something different with their sequencing as to how they grouped up. I'll interrogate them soon.