Data allocated incorrectly? Or billed in advance?

I recently signed a 20GB Data Price Plan Top Up contract, R229 per month.

When I logged into the vodacom website, I noticed a bill for 366.4, but only received 12GB (received the modem on the 13th of June).

Looking at the bill, it seams like I being billed in advance, for June+July, and the 12GB is pro-rata for the month at 137 and 229 for July.

When I tried to confirm this with a very condescending agent, I was informed that I am being billed for May and June instead, apparently I am being billed from the 27th of May (the day I ordered the device and data) - how can you start billing if a contract was not signed yet?

But if I am being billed in arrears as claimed, it means 8GB of data is missing, if I am being billed in advance, the missing 8GB makes sense (Pro-Rata), but the agent persisted that I received 20Gb at the start of June?

So either 1) I am being billed in advance - the agent doesn't know what shes talking about 2.) I am being billed in arrears - 8GB of data is missing.