Refusal to exchange a brand new faulty MIFI

Good day Telkom. I bought a mifi router on the 26th of September. The router has battery fault, barely last 2hours on full charger. I have monitored the device on several occasions since then. I went to the same store I bought it I was not allowed to exchange it for a new device, even though I am within the 7 days grace period. They insist on taking it for 21 days repairs, How is this possible when the grace period hasn't lapsed. The store involved is heidelberg mall Telkom. I spoke to the store Manager Phumali Mkhwa****. I’ve also tried twitter was not of any help