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 6-speculate-allow

Receive a file:

wormhole receive 6-speculate-allow

Done.

You can also rest in the fact that your data is secured by a Password-Authenticated Key Agreement (PAKE).