Hello megotate,
We understand that your shared Numbers document is showing offline while you are connected to the internet. You’re in the right place, we’d be glad to assist in seeking a solution.
Are you able to still collaborate while it showing offline? We ask because we want to isolate the issue to it actually being offline, and it showing incorrectly that is offline.
If restarting your Mac does not help, follow the steps here to see who is collaborating: About collaboration for Pages, Numbers, and Keynote
Use these steps specifically to try to refresh the file: "See who's working on the document
To see who's joined the document, tap or click the Collaborate button. If you see a dot next to someone's name in list, it means they have the document open. Tap or click the dot to see where they are editing.
If you want to communicate with other participants, you can add and reply to comments without leaving Pages, Numbers, or Keynote using threaded comments."
We hope this helps. Have a great day.