Insure my car on another name?

I just bought a new car and the dealer said with the topic insurance that is no problem. So I’m 21 and I assure the car therefore on my mother who drives on 30%. Now my name is written in the letter and in the bill, because the dealer said that would go. Do the insurance companies do that or not? Today the insurance company said that it could be more expensive now. The car is insured with the alliance, by the way, where my old car was previously insured. car was also insured (but my name was not in the letter)