I'm F4HDK, the creator of these NPR modem. The unprotected telnet access is only for local management, especially when you connect only 1 PC to one NPR Client modem. If you need security for accessing configuration/management features of the modem, then you can deactivate the telnet server inside the modem, and put a R-Pi dedicated for that feature, which will be plugged locally to the modem via USB. Then you access the R-Pi via SSH. This is explained in the "advanced user guide". But of course trafic will be kept unecrypted, due to amateur-radio regulations. (telnet is only for management-configuration).