Yes, the error messages returned by the bank aren't very helpful. If you are seeing 'Card Declined - Contact Issuing Bank' then it most likely is just a typo. Please double-check the number entered. Especially the card expiry date. Is the 3 digit code correct?
Have another go and if still no luck please contact customer service and they can provide more detailed info on the error you are seeing.
Issue Processing Mastercard 2-series BINs
In 2017 Mastercard released some new credit cards beginning with a '2'. Our current checkout page is unable to process these cards. This will be rectified early October 2017. In the meantime, please try another payment method or call customer service who will be able to process your card.