Change an appointment with KPN

I need o change an appointment with KPN. Problems with the internet conexion between the street ans my house

The problem is that, when I call to KPN to change it, they tell me that they can not do this because I am not their client so they can not see my data, the solution is call to Tmobile to solve it.

And when I call to Tmobile they tell me that I have to call to KPN to change the appointment, because they cannot do that

And now,after two days trying to change an appointment I am in the same situation... I don´t know what to do
Any idea?