Which instructions are you following? Any chance it's this link?
https://github.com/MakiseKurisu/NVG589/wiki/Root-Access

I don't understand all of the instructions because I know dangerously little about Linux. But it looks (to my uneducated eye) like instruction 3 tries to enable a a telnet daemon on port 9999. What is the output from that command? Does it look like it succeeds?

If you say that you can connect to 9999 from the current session, but not from another session, I wonder if it's missing a step which enables the daemon for all users rather than just the one that's logged in. Maybe the instructions are missing some kind of elevation command before enabling the daemon on 9999. Or maybe a simple "sudo" is needed there?

Or maybe port 9999 is firewalled on that model of router, and you have to do more than just start a telnet daemon?

These are all wild guesses, or you may have already tried all those.