I need to downgrade my ios 14 to back ios13

See if anything here helps.

Bluetooth - If you can't connect a Bluetooth accessory to your iPhone, iPad, or iPod touch

Bluetooth FAQ

Bluetooth Quick Assist

Bluetooth Wireless Problems

Bluetooth devices become unresponsive

Bluetooth Interference Sources

Bluetooth Interference Sources (2)

If you are using Wi-Fi, see if your Wi-Fi is capable of setting up a 5 GHz signal.

Apple does not support downgrading iOS or iPadOS unless it is a beta version of software.