iPad 10 has demo lock despite not being a demo device

Hello, I have an iPad 10 with one problem. It was put in demo mode, and I decided to reset it to factory settings. (I remind that it is completely my property, I have a receipt and a box from it with a serial number, which is indicated on the back cover/activation screen) Now it is stuck on a white background with the inscription "Property Of Authorized Apple Reseller" without its possible to configure it. How can I remove it and return my iPad to normal condition? It is definitely not the property of the store, it is definitely not stolen and the seller definitely did not deceive me, I checked everything thoroughly. Thank. (Sorry for bad English)