heat relay board is under the platen. taking off the back cover might be easier since there isn't a heater attached to it. it'll be the only PCB there, but it's screwed in. If you have a tiny screwdriver life is easier, but worst case scenario like me, it's a screwdriver bit and some pliers to loosen them off.
There is also the heater control board, in the back with all the other boards. Barring a cable getting stripped or cut somewhere along the way, thats the entirety of the heat system, ruling out a really rare failure of the heat element itself.
Which of the 2 is the more likely is hard to tell. If your shop is quiet, you can hear the click as the relays engage on the relay board, turning the heaters off and on should make them click. i'd imagine the relay for the dryer just isn't clicking on, but i've been surprised before.