External ISDN modem?
When I had ISDN the "external modem" I used had the networking setup within it. I only had to plug a standard network cable from its RJ45 out to the RJ45 in of the network card on my PC. I then configured the PC to be a DHCP client and the "modem" provided the IP address.
In other words there was no modem configuration step on the PC itself. It was all done in the modem. The setup on the PC was no different than if I had it in the office or on a cable "modem".
The ISDN "modem" had "circuit IDs" that looked suspiciously like phone numbers. As I recall the setup on the "modem" had to be the same as that at my company's had and I also recall this being a bitch to get going initially. This was mostly between the phone company and the network guys at my office.
All of that is different from serial (real) modems as for those you do have to configure setup on the box to do ppp. (Of course the modem itself has configuration as well which varies by manucfacturers.
P.S. Back then I was told ISDN stood for "It Still Does Nothing".