Are you using prepaid or postpaid plan sim?
If prepaid, just changed sim (buy new one)
If postpaid, just rant at customer service! Don't tell that you used VPN! tell them that's not your fault, and you don't know what's really happening.
all internet companies in greece have blocked-use vpn this has happened suddenly and I am now looking for a way to have unlimited free internet on my mobile phone, as I did before blocking