Register a manual cloud on second client machine

I seem to have got stuck when trying to access a manual cloud (on Linode) from a machine other than where I used Juju to create it.

On the 1st machine I used Juju 2.8.8 to add-cloud, bootstrap, add-machine and finally deploy a charm with no issues, just following the doc.

However, I can’t find a way to use the exact same version of Juju from my second machine using the same versions of everything (elementary OS 5.1.7).

So far I’ve re-done the add-cloud, but using add-credential says I don’t need to, and when I use register it asks for the controller name, then just sits doing nothing forever (I gave up after 30ish minutes).

Both Linodes are running Ubuntu 20.04.
Used “root@juju.mysupersecretdomain.dev” during add-cloud (both times).
SSH authentication is via public key, exact same key on both machines.
All working well from 1st machine, can even juju ssh 0 to end up as ubuntu user on machine.

What did I do wrong?

Do I really need to add a user to the controller etc?