I had a hard time of fruitless Googling trying to find how to fix my subwoofer. Turns out it’s very easy. Here you go. You’re welcome.
In case the URL goes bad one day, turn off the sound bar, plug in the subwoofer, stick a paperclip into the “ID Set” hole on the subwoofer and hold for five seconds, then press the mute button on the remote for five seconds, then power up the sound bar.