Why doesn't my payment gateway show at checkout when the customer shops in US dollars?

Checkout only shows a gateway when the order's currency is in that gateway's Supported Currency list. If the shopper's currency isn't on the list, the gateway is hidden for them.

To fix it:

1. Go to Settings → Store Settings → Payment Gateway and click Configure on the gateway.
2. In Supported Currency, add the extra currency, for example USD.
3. Click Submit.

You can only pick currencies that gateway supports. Razorpay offers INR and USD, Stripe offers several, and Cashfree, PayUmoney and PhonePe are INR only. If a gateway can't take that currency, enable a second gateway that can.