T-Mobile Prepaid eSim Plan

I am not sure what AT&T and Verizon offer in terms of taking international credit cards.  Countries not on the list can be due to a lot of reasons.  One of which, is fraud prevention and another, not having any banks from Singapore in the US.  Barclay's is a common one accepted across Europe and they do have banks in the US.  Their cards works from the countries listed.  However, some other banks in the same countries do not. 

When arriving in the US.  Most major airports have cell shops right in the airport.  You could always stop in a store once you land and buy a SIM or eSIM from one of them.  Amazon.com or BestBuy.om might offer more options.fpr you to purchase internationally before arriving in the US.