iPhone 12 Pro Max internet barely works on cellular or WiFi

recently purchased a 12 pro max on a contract with 02, it’s pretty tragic, internet barely works on cellular or WiFi, never any signal to make calls, not pleased with it at all, it’s not an issue with o2 network as we have another phone on 02 which is fine on the WiFi when the 12 isn’t on the networks and if we put the sim from the 12 into another phones it works absolutely fine, can we get the phone changed or fixed, seems to be a big issue with the iPhone 12 Pro Max… cheers

[Re-Titled by Moderator]