according to me, R19 and R20 don’t need to connect on servers to start, they connect once installed first, but then works fine every other starts, unlike new versions.
I say this because you can use cinema 4D starting the app over and over again offline. But with new versions, you need to be connected to the maxon server at least once every 2 weeks otherwise the app is blocked. R19 and R20 only detects same serial on local network. Hope that will help