I've also moved your post out of the 3Made Easy board and to the Prepay Plans and Services which is the best-suited board for your query 👍
Regarding your query, if your plan is set as the prepay 20 & you top up by any value greater than €20 your plan will be activated for the next 28 days. Can I get you to check your My3 account & let me know does the plan show as activated there for you?