Trouble with initial setup on RB750Gr3

Come on, guys, you all sound like you actually read instructions before powering up any new, unknown device and start fiddling with it.

If that was the case, then It wouldn't be such a common meme, besides the obligatory xkcd's:
https://xkcd.com/293/
https://xkcd.com/2724/

@mikrot
Point #22 of (suggested) Good Practice and Common Sense Advice:

The OP's reported experience is not so rare, the Rules of the Mikrotik Club and the GP & CSA were written exactly because similar experiences are common enough.