OK. after pulling up your model the problem is the defrost thermostat, this is a common failure on these. its part # XXXXX on this diagram.what you need to do is take the freezer apart again and while this is still cold (it has to be cold to test) unplug it and test it for continuity. it should have continuity. if not replace it. theres a link below where you can order it. this runs the defrost heater and if its bad this will not defrost so the problem will come back after a few weeks.
link to part
thanks and best wishes, Bryan