MCC: Can't play with friend due to network issues

My friend and I started playing through Halo CE on MCC up until recently. Now he can’t connect to any online elements on the MCC and the network tab under the settings read ‘unavailable’ for him. We’ve tried several fixes, none of which have worked so far.
Our NAT types are open. Everytime I’ve tried to join him it eventually ends up with a ‘connection failed’.
Any help would be appreciated!