Put the probe in the cool end set to around 84f. Put a digital thermometer probe under the basking spot to check if its reaching 110-115f. If it is lower, move the basking spot higher or turn the Habistat up a few farenheit.
You mention that your beardie is on sand, how old is it? Do you know the risks of impaction? I'm not going to rant, I just want to make sure you're aware of the risks lol.
If not, do a Google search on 'bearded dragon impaction'.