The imore link posted above is for an older iOS version, I'm sure that some of the tips are still valid but it might be worth looking for more recent advice. My experiences with Bluetooth are different, I've used it in loads of hire cars, regularly use it with a Bose headset, different JBL speakers, in my own car, always works very well. That's with an iPhone 8 running the latest iOS. I would suggest a call to the Apple helpdesk, they're the best source for solid troubleshooting info.