Hi Francesco,
Does the DHCP server and NPS server on the same machine?
If not, we need to install NPS role on the DHCP server too, NPS on DHCP server works as NPS proxy.
In DHCP&NPS proxy server:

In remote NPS server:

Shared secret is configured when we add RADIUS server & client, it is not configured on DHCP role.
Besides, here is the checklist of configuring NAP enforcement for DHCP:
https://technet.microsoft.com/en-us/library/cc772356(v=ws.10).aspx
Best Regards,
Anne
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