Try running the car on petrol and then feel if the RPMs increase more with speed constant or not. If it does, then sir i believe the CVT is dying out on you.
Remember, this transmission variant of Honda does not have a torque converter. If the problem persists, its either the belt slipping or the pulleys are not doing the job right.
If it doesnt do the increase rpm thing, then we can proceed with CNG related issues such as tunning or advancer setting/replacement.