Carrier lock

AT&T is my original carrier over 2 years ago I changed carriers to Consumer Cellular neither carrier can seem to unlock my iPhone 7plus after 9 days of trying and several phone calls and visits to AT&T stores I still am unable to switch to Spectrum mobile there has to be a solution to this problem I am sure I am not the only one who encountered this problem by not being able to switch carriers I am losing 30 dollars a month.