I checked the Roamless desktop website and they list: Wind 5G, Vodafone 5G, & Cosmote 5G for Greece. Probably a little pricy when compared to some other services, but they have been good on connectivity for me in the past for Europe. 20Gb=$35/30 days. They frequently have sales that can cut your costs substantially and the customer service has been good also.

I have had a plan for a few years with them, and it is a solid service where credits never expire. They have one & done plans also, which I have quoted above.