Should only be overheating if your ATF is old and/or low level and the torque converter is spending a lot of time unlocked i.e. you pushing the vehicle pretty hard or the solenoid controlling the lockup is not working.
The warning light only comes on when the ATF temp at the output of the torque converter reaches 150 degrees C so even a trans cooler may not fix your problem, as it sounds like you running heavy at either higher speeds, in lots of hills or there is an issue with the gearbox solenoid operation.
If you can't get a trans cooler to potentially assist managing temps, what you can do is have the ATF replaced with correct type and make sure that it is at the required level at the specified temperature (there are videos on Youtube of how to replace the ATF and get the levels right if you can't get to a mechanic/auto trans specialist).
When your doing higher speeds try to keep gearbox locked in 3rd gear (if a 4 speed auto or 4th if 5 speed) so the Torque Converter stays locked longer/is under less load and if in hills try not to put your foot flat to the floor or accelerate up hill (i.e. use minimum accelerator input to maintain speed or very slowly accelerate as that will create the least amount of heat in the torque converter (if you push the accelerator to the floor the TC will just slip more and create additional heat).
Hope this helps a bit and safe travels