Hello Ianm05,
We see you've reviewed the best article to assist with this solution, If you're left or right AirPod isn't working - Apple Support. Be sure to complete the guidance from this section:
"If there's low volume in one AirPod
If your left or right AirPod isn't playing any sound, or if the volume is too quiet, follow these steps:
- Check the microphone and speaker mesh on each AirPod.
- If there's any debris, clean your AirPods.
- Go to Settings > Accessibility > Audio/Visual > Balance, and make sure that the balance is set in the middle."
We hope this helps. Let us know if we can assist with anything else at all. Take care.