Not sure what you mean. Do you want to telnet to the router? Last time I tried telnet to a telnet service just worked in a browser without any drama or extra config.
R6400[v1] or R6400v2? Look for "Model" on the product label. Firmware version?
I don't know about an R6400[vX] with your (undisclosed) firmware version, but some models have a "debug.htm" page on the management web site which has an option to enable Telnet access; others can be provoked using an external program. I use the program which I modified: http://antinode.info/nte/ , but a Web search for terms like, say, "Netgear Telnet enable" should find others.
Netgear appears to have been removing that (undocumented, unsupported) feature from various models in recent times. Whether that's intentional or just another blunder is not obvious. See, for example:
Comment #3 by @antinode contains the answer, which is using the NTE program to enable the telnet daemon on the router. After doing so, you will be able to connect to the router's LAN interface IP address via the telnet protocol. Note that the NTE software @antinode linked to (and wrote!) is not provided by or supported by Netgear.
I can confirm that NTE version 0.5 successfully enables telnet on the Netgear R6400v2 router running firmware V1.0.4.118_10.0.90 (which was released yesterday July 30, 2021). On the Netgear R6400v2 router running firmware V1.0.4.106_10.0.80 or older, one could enable telnet via a web browser by navigating to http://192.168.1.1/debug.htm (replace 192.168.1.1 if not using default LAN IP), authenticating if necessary, and then checking the box next to "Enable Telnet." In firmware V1.0.4.118_10.0.90 the "Enable Telnet" box is absent from the default.htm page.
