False advertising...battery useless

Due to unforeseen circumstances I found myself having to buy a new phone and a new contract. What was advertised was what I received... Except the 4 days of battery life. The battery size is right. But what was advertised was "up to 4 days" of usage. The phone when fully charged only allows 12 hours..and that's on the "Ultimate battery saving" setting that you have to Google for as it's not in the manuel and well hidden if you press certain buttons in a certain sequence. In standby with the phone screen off...it will lose power very quickly This is on every battery saving setting possible.

Now I have to walk around with a charger just in case ?