CI3 | Connect to remote database

Hello,
I have a web application with its own MariaDB database. But I also have to connect to another 2 databases that run on clients server (on premise server, normally not connected to internet). 

He told me, there are two ways how I could connect from my app to his server:
1. He enables some port of the server to be accessible from the internet 
2. Connecting to the server via VPN.

Once I will be connected to his server, I access the databases, run some stored procedures and get the result back to my server.
There are two different databases, one is Firebird and the second is MS SQL.

Is this even possible? How would you approach this problem? What way of connecting to the server would you prefer?

Btw, I need a real-time connection. 

Thank you! I will appreciate any suggestions.