Adding Windows Modern device error
Look at the warning/error message(s) shown on the device screen, or your MobiControl server log files. if your device had already successfully got connected to the server but encountered some problem, there should be log entries giving more details.
Have you successfully enrolled any devices of other platforms (e.g. Android+, Android Enterprise, iOS unsupervised, etc.) onto your server. If not, you might first try with an Android device, which is relatively less restrictive in terms of network and security requirements. If you can get an Android device enrolled, you need to solve it first, before you proceed with Win10. Anyway, this is the simplest debug flow I would follow if I were you.