While setting up Opening Balances, I entered an amount under ‘Accounts Receivable(AR)’. But, I received the payments for the same on a later date. How can I apply this amount to the AR?

No, you will not be able to apply the amount after entering the opening balance.

If you have entered an amount for AR in the Opening Balance, you also need to import / create invoices amounting to the value entered in the Opening Balance for AR. You can then record the payment for that invoice.