MTN figure your **** out

On the 28th of march I received my new router today the 3rd of April I receive a billing amount r1400 which is confusing coz I just joined and I paid R850 something when I first called them... So now I have to pay another R1400 before the 4th for no reason so please help me understand why I need to pay almost over R2k just for a device I've been using for 6 days