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Hello lukasz284,

Thank you for reaching out to Apple Support Communities.

Lets review these steps:

If you still can't pair or connect your Bluetooth accessory

1. If you previously connected your Bluetooth accessory to your iOS or iPadOS device, unpair the accessory, put it back in discovery mode, then try to pair and connect it again.
2. If you can pair your accessory with some devices but not your iOS or iPadOS device, unpair the accessory from your other devices. Then try to pair with your iOS or iPadOS device again.
3. Check with the accessory manufacturer to make sure that your accessory supports your iOS or iPadOS device.
4. If you still have any of these issues, contact Apple Support: 

• You can’t turn on Bluetooth or the setting is grayed out.
• You can’t connect any Bluetooth accessories with your device.
• The accessory manufacturer confirmed that your accessory is working correctly.

More details can be found here: If you can't connect a Bluetooth accessory to your iPhone, iPad, or iPod touch

If the issue persists, are you able to pair any other device to your iPhone? Have you tried unpairing with your other iPhone? Also, what is your exact iOS version?

Good Luck!