@dimitris89

I have the same issue,  My ISP has installed a custom firmware but I want to revert back to default TP-Link firmware. I need to get into telnet. But it says the Have failed for * times,

@the_giraffe 

First of all lets clarify tha TD-W8961N v3 has working Telnet. It is written on the specs as well.

But some ISP on many countries have custom firmwares (in this case it is not a unbranded w8961n) and they have set a custom passwords (hardcoded to the configuration) to access the telnet service (and it is not the web gui admin password). Unfortunately, the end user does not know that “hardcoded” (in the firmware) password as usually ISP doesn’t want him/her to mess around with cli commands. 

To reveal it (if you cannon find it using google), it takes some skills and some risk but it is quite easy.

First you need to save the configuration usinf web gui (Menu)maintenance->(tab)Firmware->(button) RomFile Save. This saves a ROM-0 file which contains the configuration of the router in a compressed binary form (so no you cannot open it with notepad). In this file the custom ISP password is stored (and all other conf options). All you need is to “decompess” this rom-0 file. Search the internet to find a tool for this purpose. Mind than some of them are recognized as virus/harmful from Microsoft or other antivirus (defender) when you try to download them. If you are very sure they are not harmful or find one that is not recognised as harmful, use them to decompress the file and the password will be reviled to you. This procedure is common to other routers beside tp-link that use same or similar rom-0 configuration backup files.

Once you use the (recovered) password to enter telnet you can change it to a new one by using the command

TP-LINK> sys password thisismynewpassword (use whatever you want)

whatever you do you do it at your own risk! Please note that this procedure could void your warranty or support or could brike (brake) your equipment. If you do not understand the risks don’t try it. If the equipment is not yours (belongs to the isp/carrier) don't try it also. 

@Keysmith 

Thank you so much for the clear answer :) I will follow it