Remove the coils from the RTA and put it on your mod. Hit the fire button and see if it gives you the "too low" error message. If it gives you the message with no coils installed, the RTA is shorted.
If it tells you "check atomizer" without the coils installed, then you may be building the RTA at a resistance too low for the mod.
Remember, with three of the same coils you must divide the resistance of one coil by 3 to get the actual resistance seen by the mod. So, if you put three 0.3-ohm coils on the Azeroth, the end result will be 0.3 divided by 3 or 0.1-ohm.
If your mod will not fire at or below 0.1-ohm, you will get the "too low" error message.
If you don't understand what I just posted - you might want to read up on Ohm's Law to understand what is happening - though it's good that you are using a regulated mod to prevent any harm.
Not trying to be condescending - just want things to work out for you, safely.