Freezer works fine but fridge side stays at about 45 degrees

First, I think it’s important to know that the Ref side has no cooling coils, it’s basically a box and the cold air in the freezer is blown over by a fan through a channel into the Ref side. This channel has a door which regulates how much cold air goes through. You probably have two control knobs in the Ref side, one controls the temp with numbers 1-9, 9 being the coldest. This temp control tells the compressor how long to run. The other control is for that little door and it may have letters A-E, E being the coldest. If you set it on E (coldest) then this will close that little door and keep most the cold air in the freezer.

Now, why is that cold air not getting over to the Ref side?
1. The little door is not open.
check your control setting. You should be able to feel air coming through the channel on the Ref side.

2. The fan in the freezer is not working.
You can hear this fan running when you open the freezer.

3. The coils on the freezer side are clogged with ice which won't let the fan circulate the air over to the Ref side.
This is a failure of the defrost system.