Good day

I have a RouterBOARD 750UP, I've Hard reset the unit, log-in and select 'No Default Configuration' and then it reboots. After reboot, I can't get back in with the MAC Address.
Then after restart the eth port 2 to 5 just goes ON and OFF every 10 sec. Then I have Hard Reset the device again and nothing, I left it off for a day. reset it again and then I got in but with the default
configuration.

If I go in the terminal and type ' /system reset-configuration' then it reboots but then no MAC Adress. can't get in to set unit.

What Can I Do?

You can try netinstall! But this router is so old!