Go to the page http://192.168.1.254/cgi-bin/etherlan.ha and
login if necessary. After the page loads, view the source of the page and search for "nonce". There should be something like this:
<input type="hidden" name="nonce" value="815a0aaa0000176012db85d7d7cac9b31e749a44b6551d02" />
Copy the long string of text into the nonce field below. For this example, you'd copy `815a0aaa0000176012db85d7d7cac9b31e749a44b6551d02` into the below text field.
Push Save to enable telnet. You should see a page on your modem with the "Changes Saved" on it. If you don't see this, it didn't work.
See The full article for more details and what to do next.