Here is the parts breakdown Frigidaire GLRT212IDW1
Take a look in the Shelves section Item 23. Item is listed at the bottom of the page as F23. Click on the part number (blue print) and quite often another page will open, scroll down and often there is help from others who have used this part.
Leave the covers off the freezer coils
Keep the door shut so freezer stays cold
Leave your timer where you can get at it
You will see a couple raise ridges on the timers cam shaft
Use a screw driver or a dime to rotate the cam till the freezer fan goes off
It only rotates in one direction so do not force it
Also be careful, you do not want to get a shock. You could unplug fridge move it some, plug fridge in till you get to defrost. On some timers you can hear/feel it when the contacts switch
You are now in a defrost mode
Check to see if your heater comes on
It should, the only caveat is that the defrost thermostat must be cold
It opens at warmer temps to prevent too much heat in the coil area
If you have no heat then it probably is the heater itself or the defrost thermostat. | 
