iPhone US Reseller Flex Policy

My father received an iPhone SE (16 GB) as a gift some days ago. We're in India and the phone has been purchased from the USA. While doing activation we're seeing the error, "SIM Not Supported". (Detailed message: 'The SIM card that you currently have installed in this iPhone is from a carrier that is not supported under the activation policy that is currently assigned by the activation server. This is not a hardware issue with the iPhone. Please insert another SIM card from a supported carrier or request that this iPhone be unlocked by your carrier. Please contact Apple for more information.')

After doing a lot of research, I found out that the phone is under US Reseller Flex Policy.

I don't have the contact details of the seller as the item we have received is a gift.

Can anybody suggest what can be done?

Is there a way in which I can perform the activation in India and continue using the phone normally here?