How to reconnect Bluetooth if I hit forget device

The same way you connected it the first time. Set the bluetooth device to pairing or discoverable mode as indicated in its own instructions, and look for it in Bluetooth settings in your iPhone.

More details here: Pair a third-party Bluetooth accessory with your iPhone, iPad, or iPod touch - Apple Support

You need to put the BT accessory in "discovery" mode (as if it were new), as per this support article:--> Pair a third-party Bluetooth accessory with your iPhone, iPad, or iPod touch - Apple Support

Regards

Giulio