Defect iPhone 6s that won't be fixed/replaced

Recently purchased an iPhone 6s CPO from the iStore in Sandton City. The device has been faulty since I purchased it. The signal kept dropping/SIM card wouldn't pick up. I returned the device the same day.

My device has been in service twice and the issue still persists. The service technicians have run all the tests and say the device is 100% fine which is bull... No one seems to be doing anything about this issue at the iStore.