There is a bug in OvrC were Telnet doesn't connect to devices via OvrC. Here is a workaround

In OvrC, go to the site in question and navigate to Site Settings

Click on Network Ports Scanned

Change the Telnet default port scan (Port 23) to, 'Other'. Press Save, return to the site and perform a Network Scan. The Network Scan MUST complete on its own. 

After the Scan, click on the device that you would like to Telnet to, and navigate to the Connection icon to list the connection options

Telnet will show the option for 'Open Connection'. Once clicked an OvrC connect link will be generated

Copy the URL and open PuTTY

Paste the link in the Host Name and put the port number supplied in OvrC in the Port field removing the default '23'', ensure the option 'Telnet' is selected. and press Enter

The Telnet session will initiate and you can use the CLI to send commands to the resource.