Today I encountered the issue where I added a VPS to a private network in the TransIP dashboard. I followed their blogpost on how to configure the internal IP address found here.
However, after running
I still didn't see the eth1 interface up.
In the control panel I saw that an error was presented on the login screen:
ACPI: Error installing CMOS-RTC region handler
I added another VPS to the Private Network which then got the same error. After searching, I found this blogpost. This added up to my suspicion that it has to do with the configuration of the internal interface and it not being up. To make a long story short:
A VPS reboot fixed the issue and the error is gone. The interface is now up.
Note that you can reboot your TransIP VPS by clicking 'reset'. Not the best naming for a button that restarts your VPS, but that's a whole different story (right Donald A. Norman?).