grindstone is right about where the decline is happening, and it's worth understanding because it changes what you can do about it.
The tools you're describing are in a category that attracts a lot of disputes, so their payment providers run tight rules. A consumer debit card from a country the merchant sees a lot of fraud from, with a billing address that doesn't match the signup details, gets refused before your bank ever sees the attempt. That's why the bank truthfully says nothing is blocked.
Things that genuinely help: make the billing name and address match exactly what's on the card, use the same email domain across signups, and don't retry the same card six times in a row — after two or three attempts you're on a velocity rule and nothing will work for a day.
If it keeps happening, the usual answer is a card issued for exactly this purpose rather than your personal one. The Crypto Cards & Prepaid board here is where people compare what currently gets approved on which merchants, and it moves month to month, so it's worth reading there rather than taking a single recommendation from anyone.
One caveat: some of these merchants block whole card ranges, and no amount of getting the details right beats that.