In August/September 2021 Codemasters broke something with Racenet for Dirt4 players on Xbox. See https://imgur.com/gallery/gS2DaPo
Xbox players will not be able to connect to Racenet until Codemasters fix their problem, (PC and Playstation are still fine). All of the Event achievements* are affected and if you need them to complete, complain to Codemasters on their forum, reddit, twitter and via their customer service email. They will tell you to go troubleshoot your connection but see the imgur link above, literally no-one on Xbox can connect to Racenet in Dirt4.
*achievements Delta Force, Flavour of the Week, and Dirty Dozen