Default Galaxy s4 i537

Hello, having issue with a null/null IMEI after factory resetting the phone. Also the service menu is blank so there are no options. I have researched a lot but am not finding a solution for imei that either I comprehend or works. Im not amazing smart with these things, but with some help I know I can get it fixed. I bought this phone and got screwed over my own fault just looking to fix it