wco81, 3 started to offer prepaid packages with EU-Roaming
https://www.drei.at/portal/de/privat...3-mit-roaming/

The smallest package is €15 for 3000 Minutes, 300 Minutes from Austria to the EU, 3000 SMS and 4GB Data EU-Wide.

For T-Mobile Prepaid (Named klax) you'd have to buy additional packages for EU-Roaming which are not really worth it.

A1 for sure has the best network coverage on the countryside.

Don't worry most post offices usually are well stocked as they are also selling Internet, Landline and TV products from A1.