Pee pads (or any other station that you set up in the home) is a big no no. It is very hard to break the habit of using pee pads, and rightfully so, you have been enforcing that it is ok for your dog to eliminate in the home.
Start over. Get rid of the pee pads in the home. You will also need to pick up anything that looks like a pee pad (I.e. Bathroom mat, rugs,etc.) You can still put one outside, since your dog now thinks that is where he/she must go.
Do frequent potty breaks at first, and gradually build up.
Reward your dog and make a big deal when they go potty outside. BUT, do not react when they have an accident inside, and Yes, it will happen... multiple times.
It can take months to break "pee pad training"
Keep up with it!!
It is recommended to use a crate when potty training as well.