Bluetooth

Try resetting network settings:

Settings>General>Reset>Reset Network Settings, keeping in mind that this procedure also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you've used before, but NO OTHER data is affected.

When done, restart your iphone.

If the issue still persists, since your device is still covered by warranty, have it inspected at an Apple Store or AASP:

Apple – Location Selector

Regards

Giulio

If the Bluetooth device support a Bluetooth profile the ios device supported

https://en.m.wikipedia.org/wiki/List_of_Bluetooth_profiles

most Bluetooth devices needs to be put into pairing mode to connect to new devices the manual of said Bluetooth devices or the site of the manufacturer should describe one goes about putting them into that