Networking - Static IP - DHCP by default

I have a Beagle Bone Black,

I am running debian 9.5.

I am new to linux but fumbling around enough to get mono and a few other things working.
My problem is networking.

I have tried to set static IP using etc/network/interfaces, however something else is configuring my interfaces.
Even when I comment all the lines out in the interfaces file, i still end up with a DHCP server on Eth0, loop back mode enabled and specific USB interfaces.

How can I find where these are being setup, so that I can override them? I am not running anything like Network Manager or alike.

google connman

connmanctl takes over ownership of the network adapters. There are two ways to fix this. One is to alter the startup scripts so that connman no longer runs, and you can use the interfaces file to configure the network. This can be done by mounting the SD card on a PC and editing files in /etc/init.d

Or, on a running bone system with serial console connection, you can run connmanctl from the command line and it will allow you to make the configuration.