Hello SumitThakor,
Welcome to Apple Support Communities. We understand that when you connect your headphones to your iPhone using the lightning connector, the sound is mono and not stereo. We’re happy to help.
Are you using Apple EarPods? If not, which headphones are you using? Please go to Settings > Accessibility > Audio & Visual > Mono Audio. Is Mono Audio toggled on?
Here’s more information on customizing your headphones: Customize headphone audio levels on your iPhone or iPad - Apple Support
If no issue resolution, we recommend contacting Apple Support here: Apple - Support - Product Selection
Let us know if you have further questions.
Take care.