First off, welcome to COL!
Pups will only ask to go outside once they know they shouldn't go inside. She's getting the idea, but the penny hasn't quite dropped yet, so I'd be straight back to toilet training basics. If she's going right inside the back door, then it sounds like you're not around when she needs you to be. Pups are pretty consistent, so keep a record of when she 'performs' outside and be more vigilant. That and a lots of praise and patience should pay dividends.