I paid for this phone in FULL. No contract. Why can I not install my own OS?

The ATT store salesman convinced me to buy ATT insurance instead of Apple Care for my Daughter's i5s phone... "If it breaks, you pay a fee ($149) and we send you A NEW phone".  Surprise... they send you broken 'certified like new' phones.  the certified phone came with a loose screen - clearly broke

I have been an ATT customer for well over 10 years.  For many years now I have been having sim card issues.  I keep getting new sim cards and have even bought new phones from different manufacturers.  Still have the same issues.  Sim cards work off and on.  Why does ATT still used outdated tech like

Simple question: How do I view cellphone and plan prices with nothing to do with DirectTV on your website? https://www.att.com/cellphones/samsung/galaxy-note8.html#sku=sku8440478  This is insane. When I purchased my last Jaguar, I did not have to look at vacuum cleaners or other irrelevant items!I w