Factory reset the Roku. Then after it reboots cleanly, unplug it. Now go to Roku.com log in and remove that device from your account. Now plug back in the Roku and go through the steps to reregister it under your account. Now enable developer mode on the Roku once you can. Once the Roku reboots with developer mode ready, test it. Do not telnet first. You have to first log into the Roku web portal before that can work.http://ip.of.roku.here
and type that in a web browser.
Login: rokudev - Pass: <your-password-not-pass-sentence>
After that, it should work. Grant you access to the portal. If it does not, you have bigger issues. Roku may have marked you either as:
a) A developer of Illegal software on their device
b) A user of illegal software who repeatedly has illegal software removed from their device
Not just developers use the portal. Others do it just for the piracy apps they can now sideload from zips onto their Roku. As pirates proliferate the platform and cause problems in territories it becomes necessary to stem the tide. Roku now blacklists users and all the devices if they repeatedly insist on pirating with their Roku devices. This doesnt kill your Roku like a remote death switch. It just makes it so developer portal no longer works. This might be your scenario. If so, you may be able to talk to Roku.Co higher ups and explain your sitation and why this occured.