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Update: I tried to buy a prepaid mastercard from Bitrefill again, this time from a Ramnode cloud server in Atlanta. Even though I used Firefox on the server, I got the same result: it still says I'm using a VPN.
I also tried to use Vultr, but with no luck. I'm getting the same message. But even after managing to buy them, you would probably still need to submit your documents on the card's provider website.