Freezer not freezing & Fridge not cooling like they should Frost on Coils

My GE side by side refrigerator/freezer stopped cooling in fridge or freezing in freezer like it should. Removed back panel in freezer compartment and noticed the coils are frosted over. I replaced the dual element defrost heater and it didn’t fix the problem. I did notice that the defrost heater does come on after the coils are almost completely frosted over. Any suggestions what I should try next to fix this? Is there a way to test the rest of the defrost components to figure out the exact faulty part? If yes then could you please give me step by step instructions on how to test the parts? With the price of shipping I would hate to have to keep ordering different parts not knowing if that will fix it. I do have a basic cheap Cen-Tech P35761 digital multimeter if it would help to test with it. All reply’s are greatly appreciated.

It seems like the drawer in the lower part of the freezer and the drawers in the lower part of the refrigerator are a lot colder than it is in the upper shelves of both.