You need to go into your network settings and take the network search off auto and manually choose a network based in Greece.
On auto, the phone will find the strongest signal which isn't always the most suitable.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion.
This is not Customer Service so we are unable to help with account specific issues.Currently using:
Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)