You're subwoofer needs to be amplified separately,
Almost any AV Receiver needs that. Do you have an old stereo amp? if so you can use that as A subwoofer Amplifier. If not you could Put a filter in the stereo speakers with the sub.
So it only gives low frequencies but I don’t know how technical in this part you are. Don’t take that the wrong way.
Edit there are devices that do the filter job, but to be honest i think buy a different sub is the better choice on the long run.