How do I use a bluetooth remote shutter or selfie stick with ProCamera?

Please make sure you have Volume Buttons turned on by opening the Control Panel > Settings > Volume Buttons > On.

On principle, you can trigger ProCamera's shutter remotely via bluetooth if your third-party bluetooth shutter emulates a headset.

Also, please make sure you have the volume of your iPhone set to a mid-range position, not to minimum and not to maximum. 
(In case you were using a head set or AirPods, please make sure volume is set to a mid-range position there as well.)