By default, the unit hostname is stellarmate.local
From StellarMate App, go to My Mates then select your unit. The hostname is the unit name. If you changed the unit name, then just use the name followed by .local
For example, if you changed the unit name to observatory, then your new hostname is observatory.local
Another way to find the hostname is to open the Web Manager from the StellarMate App and you will see the unit hostname in the browser address bar.