Is there a Host Setting I missed that needs to be set to enable access from a mobile device?
No, there's no such setting.
I forgot to add that I'm using desktop Version 18.104.22.168 and the latest Android version on both devices, and that I'm trying to connect using an Internet ID, which I am able to do with no problem from a desktop Viewer.
We'll have to wait until the next mobile update is out, because currently there's no logging in the mobile Viewer which would help us diagnose the issue. Sorry for that.