Swich over from ADSL to fiber

I app**** for a fiber line in Dec 2017, I told the sales agent that I have ADSL and when will the fiber be active so that the DSL line can be cancelled in time and she could not give me a date so I told her the fiber must only be activated when the DSL is cancel, last week (3 months later) the fiber was installed this week I received a sms saying that the fiber is active, now I must pay for both and from that start I told them only when the DSL is cancelled, A DSL line has a 30days notice period so how could I plan this and why must I now have both active and the same time? DSL was cancel a day after fiber was installed.