As there are two issues here.
- One is the signing of the .inf and the .cat combi.
- The other is the use op DpInst.
DpInst is from MicroSoft and in the years from 2007 to 2017 they changed their ideas how it should work.
More can be found in:
https://msdn.microsoft.com/en-us/windows/hardware/drivers/install/device-and-driver-installation-software
Seems that DPInst is no longer advised as tool to install device driver.