I’ve never tried the Remote Home Assistant integration. I would assume that you need that before anything else would work.
While the idea of linking HA instances together seems great at first to me, I came to realize long before I ever implemented it, that I have a desire for each of my HA instances to be fully independent given that they are used for my home itself and then rental properties. I don’t want the possibility of one of my HA units in a rental property to have access to my home or other HA instances.
Instead what I do is manage them all independently and develop blueprints for common automations where I keep doing the same sort of thing in each system. The code management is primarily done by way of my Rental Control integration that I wrote and maintain for it driving the slots in Keymaster.