My account was blocked for months and I kept nagging them every now and then on their customer service email.
They told they were working on a process for foreign customers.

Last week I nagged them again and received a reply with a form to print, fill out and email them back with a copy of my ID and the card.
I also had to transfer 1€ with a text string in the description to the card IBAN.

Yesterday I received an email that the card was unlocked and I was able to purchase online with the card.
Of the 700€ I had on hold there, I have spent yesterday 600€

For me it worked ...

This is for mBank cards only.