You can't share over Bluetooth, only over WiFi and yes, has to the same network. If that's not possible, then you can use other solutions to remotely access the Mini.
To be clear: Out of the box, Screen Sharing, File Sharing, etc., are not enabled. A trip to the System Preferences --> Sharing control panel will turn that on. (the "same network" thing Raz0rEdge mentioned isn't correct, unless I'm misunderstanding).
You may be able to SSH into the box (if you know the IP and there has to be a user account set up already) remotely and enable that... There are ways to do it... but definitely requires hands-on first. Maybe have it shipped to where you are, configure it, and then ship it to where you need it to be? Install a "What is my IP" app that sends the current dynamic allocation to your email, yadda yadda yadda.
hmm..... but no. Built-in screen sharing still requires a user to accept the request.
So you'll hae to configure something like TeamViewer or any of the other remote collaboration apps...
Is there no-one at the home base who could take some telephone instruction to get you in once, after which you can configure things as you like?