That's absurd. You haven't used your phone for 8 hours, so many of your apps are running ridiculously long in the background.
I am currently having an issue with snapchat going in the background, over the past 6 days it's been 3 hours in background. I can barely get 5 hours of use from my iPhone 6.
Anyway, your plus is defective. Take it back and get a new one.
And the thing with signal strength, that isn't an issue if you have 2 bars. Low signal starts occurring if you are at No Service, occasionally grabbing a bar but dropping back off. My whole house has -120 to -110DBM of LTE yet I hardly ever get low signal in battery usage. It still probably plays a roll in the drain, but I still got 6-7 hours (before snapchat started acting up)