Where did you get this beta FW for RE305 from - link ? Why do you think device broken, any LEDs lit/blinking when booting ? Was that FW for the same region and HW version as your RE305 ?

In this scenario the router is actually still in router mode. Just see the link I've sent - you won't be switching to AP mode through the WebGUI of the C6 (i.e. you'll still have the Parental control...

Yep, do that at your convenience. Did you happen to upgrade your C6 FW soon ?

Nice topology diagram ! How close are the ISP router and your C6 ? It is possible that issue has emerged due to a wireless interference between those two. Have you tried to disable ISP router Wi-Fi...

@Codru Have you tried to tweak these settings (Advanced=>Network=>Internet, scroll down and select Advanced):

No, it's not a normal/reported behavior. It should just reboot, but not reset to factory defaults. I understand you're trying to do everything you can before RMA the device, but that's just not the...