To telnet into a Class I router, assuming they are running one of the firmwares which can have the Telnet backdoor ports blocked by default (see https://www.routertech.org/pages.php?page=18 ):
Enable the Dumb Terminal (TCP Port 254) and/or Smart Terminal (TCP Port 255). You can do this if you go to http://10.0.0.2/doc/special.htm or http://10.0.0.2/doc/hidden.htm or use my backup utility. Submit, Save & Reboot and then you can Telnet on port 254.
Once you have enabled the terminal(s) you can telnet into the router by typing the following at the command line:telnet 10.0.0.2 254
The router command line interface is fairly basic so you don’t get more than stats and rudimentary commands.