Sleep apnea tends to improve with weight loss, though that tends to be easier said than done. If it’s positional (e.g. limited to back sleeping) then tennis balls sewn into the back of a night shirt can “persuade” a person to stay off their back while sleeping. Finally, surgeons often recommend surgery for sleep apnea, though the long-term results of that aren’t impressive, IMO.
The dental device is pretty easy to use, FWIW.