BR500 reset after power failure

I did reset to factory defaults, re configure the router, but DHCP was still assigning the same IP address to two different devices (with different MAC address), and this for 1 to 3 IPs (6 devices, not always the same)

I downgraded the firmware to 5.7.0.4, reset to factory, re-configure, same problem.

I put back 5.8.0.1, reset to factory, re-configure again, same problem.

Then during the night there was a loss of power, so I just left it off for the night, then next day, power on, the router was reset to factory! I tried to re-load configuration from a backup, but it refused the backup with a multitude of 'error pop up windows', so re-configure manually, reboot and try again the backup, it seemed to work, but sitll with the DHCP problem and slow, veerrrry slow detection of connected devices ( like a minute or so).

Don't know what to do now, I have an R8000 (works great!), GC110 and GC510P switches, but this router just gave headaches so far, I thinking about returning it and buy another brand if I can not solve this issue.

Thanks in advance,

Mike

P.S. I was thinking of using the router (3 of them) for the office as an upgrade, but I have to admit I have to re-think about it now. I do not want to bring this sort of problem to the shop.