I know this one!
If you go to the search function, paste in [email protected] then search, your post pops up immediately, right? But you have to wait a few seconds for this instance to sync with the other instance (not sure about the technical details here) and the link to the community will eventually appear.
I tried it just now and it took maybe 7 seconds for the community to appear. Then you click that link, open the sidebar of that community, and subscribe.