Sorry but cannot find any info on the unit.

If the thermostat you refer to is then cycling thermostat then it is a possibility re: it's contacts are welded together. You could try heating it up on a stove element and checking it with a meter to see if they open. I use my soldering gun. Just be careful that you do not burn yourself.

Another possibility would be a grounded element.
Unplug the unit.
Remove both connections to the heating element.
Measure the element with meter. Usually 10 to 15 ohms.
Then measure from each heater connector to the case/frame. Both should be infinite ohms (open). If not you have a grounded element.
The element has either sagged or broken and is touching the case so that the thermostats cannot control it.