It's the thumb, and index finger crossed, the thumb on the upper teeth, and index finger on the lower teeth, and slowly apple pressure up, down at the same time, hold 5 seconds, release pressure, and repeat. There is also Dynasplint, similar to therabite, but don't know which is cheaper. You have to do mouth exercise, especially the first several months, otherwise exercises will be of limited help in the future. Check with your doctor being you just had surgery.

It can be from radiation, but see none in your signature. The lockjaw is usually referred to as TMJ.

Good luck