We are having the same issue. Sat down today to play astroneer with my son. Both of us are seeing that the game is in offline mode at the main menu. We have tried the usual various steps, of rebooting both PCs, clearing the steam download cache, rebooting the router, checking firewall logs for dropped packets, and everything looks ok. We can play other games online, and browse the general internet fine. So far its just Astroneer itself that refuses to go online. We are checking this via the Astroneer main menu which says we are offline and to go online to get updates.
Kidlet and I would love to continue our weekly game.