I thought the whole idea of Nabu Casa was to let you access your home HA instance via the phone app, from anywhere. I have a trial account, my HA interface shows I’ve enabled Cloud and connected to Nabu Casa. And I’ve turned on “automatically connect”. I can access my HA desktop via the URL from Nabu Casa, it all seems to work.
If I use the phone app, at home over wifi, I connect to my HA server and verify that Settings shows the cloud connection. But if I turn off wif and try to connect over cellular data, I just get the error message “Unable to connect to Home Assistant”.
In the configuration menu in the Android app you can enter TWO addresses. The NabuCasa address for outside AND your internal IP address for in-home use.
I logged on to Nabu Casa, connected to HA, copied the URL from the browser bar and pasted it in as HA URL in the apps’s settings. That works. This could oviously be made simpler, or at least explained in a prominent spot on the Nabu Casa web site.