IPhone XS Max issues

I recently purchased an iPhone XS Max 512GB from the iStore. Only to find out that the phone does not charge properly. It takes very long to charge up to 100%, more than 4 hours. When I took it in to iStore they ran diagnostics and said they could not pick up any fault, as did the service Centre. And now the tech is saying it’s a software issue. For spending the type of money on this device, I should not be experiencing this type of problem within a week of my purchase. It is truly appalling service from the iStore and service Centre. They have no proper resolution for my issues experienced. I have multiple products from Apple and this is the worst experience I have had to date.