Many problems with RBLHG-5nD often come back to defaults without any reason as you can not access it either via IP or on Mac unless you re-set the factory
Is it new or does it started to behave like this recently after working for some time?
Asking this because It could be either the reset button is damaged, or rust has shorted out its PCB pads, effectively acting as if reset button was constantly pressed, inducing a constant reboot/reset to defaults loop.
Another possibilty: damaged RouterBoot.
Try this:
1.- Reset it or do whatever you’re doing to be able to access it.
2.- Go to System > Routerboard [Settings] button, and tick (enable) Force Backup Booter
3.- Apply / OK, reboot.
Test it by rebooting several times. If this fixes it, it means it was actually the main RouterBoot being damaged.