Unable to connect to the halo 5 guardians lobby service, been going on for a couple of days now. Was able to play a little bit yesterday after one of many hard resets. But no luck at all today after all manner of troubleshooting. Live in Ireland currently, internet, router, other games all fine 
Same issue here in the USA. It would appear it’s the servers and not our gear,.
When having an open NAT and a solid connection and being hard wired into my modem, I can now no longer connect to the Halo matchmaking servers. If I use wireless off of a router in my house I can get a connection, but it results in lag. I have no issues with Gears of War, or any other matchmaking. I have called my ISP, and Xbox multiple times. I didn’t have this issue at launch or during November, why now? I get a red triangle at the matchmaking screen saying I can’t connect. I have Comcast internet.
I hope its fixed soon, people shouldn’t have to do all the legwork troubleshooting and testing after they’ve bought the game. They let me play a couple games of warzone so halo is all I want to play right now 