Thank you for your interest, and thank you for trying to help me out on this!
In short terms, I was trying to access the console of the device via a serial communication, and accidentally I fried out the board. Now the camera unit, motors and the mechanics in all works perfect, but the PCB is dead
So the story was:
I ordered a RS232->TTL converter, but had no extension cable and used a USB->TTL with active 3,3 V supply to the serial port. A major mistake by me!
I checked the OEM bootlog, posted on the 5020L's OpenWRT wiki page. There's evidences that the MAC is written to the flash:
Init RF/MAC data from flash memory *****
LAN MAC: ** ** ** ** ** **
MAC_ADRH -- : 0x0000b***
MAC_ADRL -- : 0x54148***
PROC INIT OK!
So I thought I should be able to change atleast the MAC via the serial console using ifconfig. If it won't survive reboot, I could try to log with SCP protocol program for file transfer, and change the etc/config/network file (if the flash structure is Linux based, and there is a network file at all). The nano text editor won't be an option also, or by luck it should be part of the Linux based OEM firmware of the camera.
Of course that depends on the protocol, supported by the the terminal. If it is a SSH, should be OK, but using telnet will produce unknown results.
In the OEM bootlog there seems to be no traces of the mydlink number, but I assume, that the mydlink number is a server-side tag, which is being binded upon MAC reporting, when the device is being registered at the mydlink portal. If I'm correct the mydlink number is not stored inside the flash memory of the camera at all. Anyway now it doesn't really mater at all

Its all my fault.
I'm considering to by a new camera, and the one which I damaged, will wait for a second-hand one with broken mechanics, but with usable hardware on the PCB
Really sorry that I could'n find if it was possible to recover the MAC via the serial com on the camera.
The sudden MAC clone also remained a mystery.
Otherwise I like these cameras, apart of some flaws they're very well build and reliable.
Best Regards!