The WNR2000 v3 only has 4 MB flash, it's rather unlikely that you have enough space (roundabout 180 KB) to install nano on the jffs2 partition. One symptom of "no space left on the device" (ENOSPC) is that the overlay is mounted on tmpfs (in RAM) only, to allow the router to succeed booting (as a kind of emergency mode), so that would be the first thing I'd check (df -h), besides trying to avoid installing any additional packages (busybox vi might not be convenient, but it's always installed and a quite usable editor, once you've looked into its basic operations).