I have a EasySolar MPPT 150/70.
It says “system off” - how do i make it go “on”?
I have the opportunity to put it in three different modes, ON, OFF and Charger only - but nothing changes and i can’t switch to ON in the menu page.
Do you have solar connected? Is the polarity correct?
Usually in Victron Connect app the reason for the off state is mentioned.
Solar connected, yes 
Polarity… dont know. 
Hm… okay. How do i connect the app?
We just bought the place and the former owner couldn’t help.
So not a new system. It must have been working at some point.
Oh dear.
There should be a sticker somewhere on or in the unit with some information on it. Usually pin or puk code for a Bluetooth connection.
You would have to download the Victron connect app (obviously) and search for nearby BT devices.
Victron have a neat youtube channel with basics on it like connecting to the mppt.
The GX is on so the battery must be on/have power of some sort. Is there a battery voltage displayed anywhere? Do you have it connected to grid at this point?