how do you know its NOT rooted? how do you know its not a used device? this shops' dealing sound a bit shady based on your post.
I am sorry but new devices do not behave this way. to factory reset the device, its in the software menu in the settings page. scroll down to security i think (going from memory) and you have an option to factory reset.
however, I would suggest you take it back to the shop and get a new device, assuming you still have a grace period and get you money back and go elsewhere.
given their reluctance to help you, you should just ask your money back. its clear to me this phone is not brand new sealed when you got it and it sounds like a bad rom is on the device, or some wonky virus or its has some internal damage.
bottom line; if the phone does not behave properly after a factory reset, its defective period and should be exchanged. there is nothing people over the internet can do to help you as we will be just going down a rabbit hole into the abyss
and without having the device in hand to test, it could be anything causing the problems you are experiencing.