With PRTG you can monitor your device via Telnet by using the Port sensor to execute a command and then, check the response code or the text returned. Here is the manual of the Port sensor: https://www.paessler.com/manuals/prtg/port_sensor
You can also write custom scripts which use the EXE/Script Advanced sensors whose manual are here:
Please, take a look at this KB article which explains how to collect data via Telnet by using PowerShell scripts:
If your device is also SNMP-capable, you can import your own MIB file in PRTG with our tool MIB Importer: https://www.paessler.com/tools/mibimporter
Afterwards, add a new SNMP library sensor and select the oidlib file you have imported.
Then you should be able to select the different aspects of your device you would like to monitor.