You're not being dumb, that sounds like how it ought to work to me!
What worked for me was to log in to my home instance, search for the whole URL for the community, and then, after getting no results from the search, change the type of the search to All. Sometimes I would change it to Communities and then to All. Eventually after a couple tries the link should show up in the results. Click it to browse to the community and then you can subscribe. It's a bit cumbersome but it worked for me.
You're not being dumb, that sounds like how it ought to work to me!
What worked for me was to log in to my home instance, search for the whole URL for the community, and then, after getting no results from the search, change the type of the search to All. Sometimes I would change it to Communities and then to All. Eventually after a couple tries the link should show up in the results. Click it to browse to the community and then you can subscribe. It's a bit cumbersome but it worked for me.