Well the Lobby server is offline for the last 3 days now (don't want to speculate on what happened). Here's a temporary fix, but you'll have to play under a Direct IP.
In general to get the Direct IP for any server, do a search on the following site:
https://www.gametracker.com/search/halo/.
For your convenience I compiled a list of active race servers:
BFM: 108.61.100.36:2302
Lickity: 216.226.128.250:2310
Lickity Backup: 108.61.100.76:2302
-db- : 108.61.118.71:2302
-db- (Custom Edition): 66.55.149.213:2302