The DPF is supposed to automatically regenerate, but it sometimes won’t do that if the car doesn’t run at high enough speeds to generate the temperature required. That usually happens because the car is just being used for short trips and doesn’t regularly get a chance to run at higher speeds on highways or freeways. Running it at higher engine speeds in lower gears might be one way of getting the temperature up, and you could run it to 3000 rpm without any danger, but it doesn’t seem like a good way to fix the problem.
