Not sure what you'd call "very remote rural areas", but when on the main rail lines through Bavaria (Munich-Rosenheim-Salzburg-Vienna and -Rosenheim-Innsbruck-Verona) if you're in the countryside you'll often experience patchy or zero mobile coverage, never mind 4G/5G.
O2 Unlimited:
- 32,99€/month for 4G/5G capped at 3mbit with unlimited data
- 42,99€/month for 4G/5G capped at 15mbit with unlimited data
- 62,99€/month for 4G/5G up to 500mbit with with unlimited data
Vodafone:
- 79,99€/month with 4G/5G up to 500mbit with unlimted data
Telekom:
- 84,99€/month with 4G/5G up to 500mbit with unlimited data
1und1:
- 49,99€/month with 4G/5G capped at 10mbit with unlimited data
- 69,99€/month with 4G/5G up to 500mbit with unlimited data