VMG8623-T50B issue after firmware update

DAC2025

I purchased a VMG8623-T50B DSL router about a year ago to replace the limited one supplied by my ISP. It came with firmware V5.50(ABPM.6.0)C0, and has been running fine with no issues. However a couple of weeks ago I updated it to V5.50(ABPM.9.4)C0, and since then I've been experiencing the odd issue where the LEDs turn themselves off shortly after booting. They all function fine initially, then appear to be turned off as part of the config, and only return to working when the router is rebooted (before becoming disabled again after a minute or so).

I know disabling LEDs is sometimes a user-configurable option on routers, but I've hunted round in the interface and can't find anything? I also checked a backed up copy of the config, and found a few entries which looked like they might enable/disable the LEDs under "X_ZYXEL_System_Info", but none of these were set in a manner that would turn then off?

Can anyone suggest what might be going on, and how I can rectify this? If the solution is to roll-back to the earlier firmware, does anyone have a link to it as I've so far been unable to find one?!

Many thanks.