So, moving house very shortly and, given lead time on new BB install is a couple of weeks, thought it would be a good idea to set up a 4G USB modem on WAN2 to tide us over in the interim. Interestingly, every time we made a minor config change to WAN2, the router wanted a reboot. No problem with that but, after a few changes, logging back in, a message pops up to saying (verbatim) 'System did firmware recovery to 3.9.1.3 due to reboot too much times'. Didn't even realise that was a thing but was unable to back up config or flash new firmware. Every attempt at flashing resulted in the unit rebooting and still showing 3.9.1.3.

Had to resort to booting into recovery mode to flash latest firmware via TFTP. While it seems fairly stable now (and having turned off the option allowing automatic flashing of an earlier firmware in the event of multiple reboots over a short time period), editing WAN2 config and rebooting does still seem to cause problems. I'm speculating that the config file might have changed in later firmware versions such that its younger siblings can't read it properly but would really like to know why system-initiated reboots (when changing WAN2 config) trigger a fail state such that it reverted to an earlier firmware version. A corrupted internal config file or maybe it's just poor management of the SDLC.