Without a monthly membership you can't make calls, send texts, use data.
If a user doesn't buy membership for 6 consecutive months then the account will be disabled and the user needs to contact 48 support to get it reactivated.
If a user doesn't buy membership for 12 (or maybe 13?) consecutive months then the account gets deleted and cannot be recovered.
I'm pretty sure that access to the €10.99 membership remains unless the €12.99 membership is ever bought.
Gaps in monthly membership shouldn't affect that as far as I know.
But check the terms and conditions if necessary.