Problem with the termination contract

I finish my contract in February because I leave Germany and I sent the letter and back the router DSL all in time, in the end I pay the extra amount for finish the contract before one year but I still get notification in April for 5 euros, i dont know for what was that, but I paid with my another bank account ( because i dont have my German account anymore) . This month I get again a new notification in my email, they saying i need pay 4 euros this time..

Sorry but I can no understand this letter and why i need still pay every month is a mistake from the computer O2? I ignore this or what is the solution I can have? . I really get stress for this notifications every month.. I just not want receive more notifications of O2 company anymore in my Email.

This was the first infromation last month
Einmalgebühren Rücklastschriftgebühr wegen nicht erfolgreicher Abbuchung - 03.04.2019 (zzgl. 0% MwSt.) 0% 4,0000 Mahngebühr - 04.04.2019 (zzgl. 0% MwSt.) 0% 1,2000


and this is the information of this month
Einmalgebühren Rücklastschriftgebühr wegen nicht erfolgreicher Abbuchung - 08.05.2019 (zzgl. 0% MwSt.) 0% 4,0000

Thanks if somebody can help me.