Faulty phone

My daughter upgraded to an I phone 8 plus 7 months ago and she has had numerous software issues with the phone, we keep needing to take it back to an Istore as her phone continuously keeps on automatically restarting without being able to switch the phone back on, the Istore then has to wipe her phone and do a factory reset and she loses all her data. The phone is still under guarantee, surely her phone needs to be replaced?