2020
Had a go with this in late 2020. I was having issues with a Virgin Superhub, and thought I'd try an old router as a temporary workaround. This also kept dropping out with the standard Virgin firmware.
I used the Linux version of telnetenable and the strings method. Some characters appeared after the _NETGEAR part of the string, which I think I discarded. The telnet username and password really were Gearguy Geardog.
Rick's comment above was most helpful with providing firmware that would actually update from the base Virgin firmware. After that, I was able to update to the latest Firmware Version 1.2.32 (All regions except North America) on the Netgear website.
The latter wouldn't update directly by following the telnet/burnboardid/update process.
For the last 48 hours the router has been rock solid, despite several phones, several computers, a TV and various home automation devices all competing for bandwidth. Wife happy. Result!
Not bad for instructions posted more than a decade ago...