Have you tried in some way to ad money your Steam wallet and see if you can buy via your wallet? Or tired another cc if you have one?
I had the same problem a few years ago and either I waited it out or I wrote some info wrong.
Yup I tried everything, I tried to add funds "declined by my bank", tried to use virtual cc "declined by my bank" even I can't add any cc info, the last message from Steam support was that the restriccions and declines comes from my bank, also the support rep told me that they can't change any cc info, this guy told me that the CVV that I was entering was incorrect.
Thank you both, I will wait, in the meantime I will use PayPal and after some time will try my cc directly.I occasionally experienced similar problems with Steam. I could buy with a certain credit card on Steam if I used Paypal, but not by paying directly.
In all occasions, after a few days without making further attempts, the issue would solve itself.
I believe each time you get a declined message, after a while their system "blocks" you for some time, and if you keep trying that blocking period continues to increase.
If you already checked all your info (including address, phone number, postal code, ...), and everything is exactly like your bank statement, try waiting 3/5/7 days without attempting to make a purchase, and see if things start working again.