Sorry but you also forgot about EEROM and Flash Memory. Also the term used was "hard-coded" not "hardwired" -- we are dealing with firmware here, not hardware.
Most motherboards, "intelligent" devices, etc. -- including routers -- use flash memory to store their operating firmware. The system allows the flash memory to be overwritten and rebooted. That's how the firmware is upgraded. Firmware images are generally available for download from the device manufacturer's website.
The recommendation for replacement was "short of a fix". A fix is trivial, and could be implemented in-situ remotely. I would expect revised firmware images to appear at http://netis-systems.com/en/Downloads/ within a few days, but that depends on the priorities of these low-cost (approx £10) devices.