Did you ever enter your Stellarmate OS license key in the android app? Step 3.2 on the page you linked mentions it. When I first set up the app on my phone a few weeks ago, it prompted me for the license key along with my username and password. I believe the app needs to be associated with a purchased device (or in your case, the OS.) Maybe this step has changed since I first did it, but without the app being able to verify that it has a valid device, it probably won't connect. Otherwise, people could use the app to connect to any kstars/INDI implementation.

If you did enter the license key and it's still not working, I'd open a support ticket on the SM site.

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