I apologize if my post duplicates another one - I used search but could not find a suitable answer to this question here.
My problem is as follows:
I have installed about 100 small HOT SPOTs based on RB750. At the date of installation, firmware was 3.29. Everything is configured and works well.
At one point, several failures occur and need replacing with some new RB750. The new RB750 comes with version 4.17 (I'm not so sure ).
Download and install 4.17 (or 5.9). Everything goes fine, reboot and RB750 works well.
The problem - after submitting the backup configuration (made in 3.29) RB750 lost any possibility of communication. No Winbox, no ping - nothing.
Until I press for a while the RESET button. After that with default configuration, RB750 work again.
Do you have any idea what might be the cause and how to fix it (though I still have 90 devices that can be damaged any time).