Correct, DR-891 doesn’t have its own serial port. When used with FT-891 the computer will see the COM ports from the radio and for other radios DR-891 functions similarly to Digirig Lite by offering several hardware PTT options. Configured correctly DR-891 will work for your ID-5100 for basic operation similar to Signalink (audio + PTT).
Digirig Mobile does have serial port onboard and you will need to use this model when interfacing with the CAT-capable radios that don’t have their own USB interface. If you want to get a separate interface for your ID-5100, you can get Digirig Mobile in CI-V configuration. Additionally to the basic operation, this will give you the option to attempt an optional serial CAT control connection to the radio as discussed here.