Speaker can't access Backstage

I added a person to be a Speaker at my Zoom Events Webinar via the Speaker quick join option. This was great as he previously had issues logging in and the quick join meant he didn't have to log in. However, he did not have access to the Backstage area of the webinar. Me and the other Speakers (panellists) from my organisation did have access, but not this one Speaker from a different organisation. I had tried to add him the normal way but the invite email would not send. I tried making him a Co-Host but this did not help. How do I give this Speaker access to Backstage while also being quick join?

(I have re-posted this as the answer given to my previous version of this question was not relevant and I haven't had any other responses. This is not in reference to the speaker (audio) hardware on my computer.)