If you're in a weak cellular reception area, you might want to consider switching the cellular antenna off and go wi-fi only. You can still receive iMessages and use wi-fi calling if your carrier supports it. Not unusual for the battery on my phone to drain almost twice as fast if I travel through an area with spotty coverage.
It could be worth a shot but keep in mind that low signal doesn't affect things THAT much. Everyone says if you have 1 or 2 bars that automatically means you will get no longer than like 5 hours of use and you will drain like craxy on standby, making the phone dead halfway through the day without using it. That simply isn't the case.
I get anywhere from -118 to -105db of LTE in my house, I can leave my iPhone 6 at 100% and wake up in the morning with it at 96% at the very lowest, I believe. Haven't left my phone off the charger overnight in awhile. I will tonight (with WiFi off this time) and see how it does. Maybe I will prove myself wrong and look like an idiot, but that's beside the point.
Another story, I had my phone switching between 1 bar of 1x and "No Signal" overnight when I went somewhere, it drained 100%-81% overnight, I believe. (8-9 hours) not from 100%-19% like OP described.
I think OP might have a stuck process, but maybe lots of pictures are being taken on another device with the same iCloud account so photo synchronization is taking battery, maybe messages are being synced from another device as well, that uses battery too. Try backing up the phone and restoring as new for a day and see if it gets better, if it doesn't, simply restore from the backup so you get your stuff back and then from there you can tinker with settings to try and fix the issue.
