Horrible phone damaged sold to me on contract

I took an iPhone 11 Pro Max contract about a year ago with Vodacom ,the phone battery capacity was at 100, within a few months the battery life had deteriorated horribly. I don’t even use my phone that much . Earlier this year it started switching itself on and off . I went to the shop where I got the phone ( Randfontein tambotie mall) , I was told it’s out of warranty and there’s nothing they can do . I must book it in myself and pay . It hasn’t even been 2 years , im paying for a phone I can’t use and this has to be the most painful thing ever . I have been with Vodacom for so long that I can’t believe this is happening.