As an example, I’ll take the Haylou GT6 headphones, which became desynchronized and connected to the phone separately, as two different devices.
I will describe the steps in order:
1) In the Bluetooth settings, remove the headphones (forget them).
2) Place both earphones in the case.
3) Take them out of the case and press the buttons on both headphones simultaneously, hold for about 10 seconds until the white/red indicators stop flashing actively.
4) After this, you can connect the headphones to your phone.