PH help me! Halo 5 could not find server suitable...

[highway]

I like Halo. Started with it on original Xbox through to Halo 4 on 360. I loved multiplayer and got an Xbox one with Halo 5 for Xmas. I've nearly finished the campaign. I can't get online. My network is connected but I keep seeing the message 'could not find a server suitable for all members of your fire team' I'm not seeking to go online with anyone else. Online research shows this is a problem. My NAT profile is open. My isp (orange) state my service is fine and can't see any problem. Worked fine with my 360.

Anyone got an idea? I'd really like to get online?