I have recently had loads of trouble setting up a Ubiquiti UniFi USG remote user VPN, the USG requires a RADIUS server in order to function correctly, the following article covers this setup freeRADIUS Setup
Once RADIUS is setup the easy part is configuring the USG through the UniFi controller.
I spent quite some time trying to get the OpenVPN Server working on the Mikrotik Router with a Linux client, It caused some pain and I didnt want others to go through that. I have therefore written this guide, taking you from certificate creation all the way to VPN connectivity.
For this tutorial I will have SSH to my Mikrotik (you can use a winbox terminal), I have chosen not to use WinBox for the configuration as its easier to deploy this way.