Suppose I accidentally restored a firmware backup on my router, in which the contents of the /etc/config/network file is simply empty... 
Is it possible to somehow update configurations via USB? 
Using something like ducky script maybe?
I have found this video — my last-last hope. But it would be great if I could solve things in a much simpler way.
You have to ssh into the router to this, but the router has no network configs... 
That is my question: How, if at all, would it be possible to connect to a router that has no network config.
I know what needs to be changed. I do not know how to connect, because network I cannot use.
Failsafe mode or serial connection are your two most likely candidates.
Did you read the guide linked by @vgaetera? It explains how to connect to a device that does not have a working network config.
Reflashing could be an option, if it can be done through boot loader.