5G on phone purchased in USA

By the mention of Airtel, I suspect you are in India.

If that is the case, Airtel and/or Jio must send out a carrier update that enables the use of the 5G channels your iPhone can use to communicate to their towers.

At this time, it appears neither has yet enabled 5G support for iPhone.

Wireless carrier support and features for iPhone in the Middle East and India - Apple Support

Hey there,

can you please provide a screenshot about what you see exactly?

this is what you need to be able to use 5G on your iPhone:

• An iPhone 12 model or later 
• A carrier that supports 5G
• A 5G cellular plan1

If your new iPhone comes with a physical SIM or eSIM, use that SIM. If not, use the SIM card from your previous iPhone. In some cases, you need to contact your carrier to set up the SIM card from your previous iPhone for use with a 5G network.