a package change, but still being billed for previous account

At first it was good, never had any issues until additional funds wanted...or to say being pocketed. I was informed if before a new package/upgrade can take place must the old be settled...then cancelled and apply a new package. I was explained "the reason to cancel is that we have to send you the new sim card" A mail was received stating if a settlement of my account is done on or before the 28th of the month we will not attempt to debit your bank on the 1st as your account will be settled.

All of a sudden things started to go haywire....I paid the new sim pro rata and my only connectivity shows the old suspended for cancellation but the new is activated. Take in mind no internet from old sim. A day after I got my package, popped in router and viola! Where the monies being debited I checked WOW!, what's happening and BOOM!....it is only for the next month. In essence paying now for 2 services with 1 modem but the old sim has no connection.

Repeatedly got automated mail alerts that the system states I am in arrears. So how is it adding up paying for 2 services at once – does this make sense?

It should be a simple change of a package instead of another additional debit. As things stand will I not receive tis credit due to the fact another policy is in place cancelling before/after the 15th of each month!