The issue is sometimes caused by the
wrong network settings on the iOS device. This frequently happens when you re-configure your
iPhone's network settings incorrectly. You may not mean to do so, but you'll soon discover that you made a mistake when your iPhone fails to connect to a Bluetooth device. A new software update that changes your existing network settings configurations and returns the default defaults might cause the same problem. In this instance, resetting the network settings is likely to solve the problem.
To do so, go to Settings > General > Reset > Reset Network Settings
on your iPhone 8 Plus. When prompted, enter your passcode, and then wait for your iPhone to reboot and reset its network settings.
Remember to set up your networks and then turn on Bluetooth on your iPhone to see whether you can now connect.
