Forget device bluetooth

I pressed “forget this device” on my iPhone because I had a connection issue with my headphones.

I use a Lightning - Jack cable to connect my headphones (old school jack ones). And the lightning - jack needs to be connected to Bluetooth in order to work. How can o find it again? I already did the “reset network”, but the device doesn’t appear. I can’t put it in “discovery mode” since it’s a cable. What can I do? Help!!!