Lidl

[inquisitor]

Don't turn on data roaming!!! USSD does not require a data connection! I hope your phone didn't establish a data connection otherwise your credit might have been reduced significantly.
The service portal always lags behind a couple of minutes while balance inquiries by USSD (*101#) are always in realtime.

Yes, you should switch back to the volume-based plan as the subscription of the monthly flatrate will recur as long as you've enough credit left and so it may eat up your credit after leaving Germany. You can actually switch back to volume-based billing shortly after activating the monthly flatrate since this switch back will only take effect after those 30 days.

[thermos]

Noted with thanks