Double billing and Poor service via WhatsApp

I have changed my contract from RAIN LTE to Telkom LTE. It's a struggle to get damn assistance on this whatsApp USELESS chat! I'm being told I will get billed pro-rated for my Telkom contract on top of the new months amount on 01 June. Why must I pay for 2 contracts whereas I can only use 1 sim in the router???? I have paid my RAIN LTE premium on 1 May which is for the month of May and as soon as I insert the Telkom sim, I will get billed the pro-rated amount? This is a *******!! Someone better call me ASAP as I refuse to pay for 2 contracts. Pro-rate my data on the Telkom sim as I've paid for May month already