instinctively, i think they dig to cool themselves rather than to heat. furthermore, heat in nature comes from above...the sun. i'm no expert, as my pacman frog died, but i'd recommend that you add a ceramic heater 'bulb' as a top-heater.

i found that when you get up to a higher temperature, evaporation happens faster, so it's a battle to keep the tank both moist and warm in an otherwise dry and cold room.

my fire belly tank is more of an aquarium with a top and with heating and cooling on the water system. so that makes it easy to keep a temperature and humidity.