A dead simple way to send files between two linux machines not on the same network is to use a utility called Magic Wormhole. It is typically included in the standard repositories and is so simple the this blog post is going to end soon.
Send a file:
wormhole send filename
This will generate a code for you to share like
Receive a file:
wormhole receive 6-speculate-allow
You can also rest in the fact that your data is secured by a Password-Authenticated Key Agreement (PAKE).