How to open telnet port in wireless D-Link router

Some routers prevent loopback connections as a security feature. Most consumer-grade routers don't have any prohibition against it, it just doesn't work.

The reason it doesn't work is that the router implements request-answer algorithms, so is not programmed to handle the case where the answer is actually another request. In other words, the router implements the algorithm of request-answer rather than request-request-answer-answer.

I suggest trying this from outside of your network to see if you are getting the same results.

It's not possible. The stock firmware has no shell and there is no known way to put one there without replacing the firmware entirely.