I dunno about introducing ANOTHER method of potty training. One reason you might be having a little bit of trouble is bc you started out using potty pads and now you are doing outside. I would not recommend introducing ANOTHER new method, like bells. My vet told me to pick a method and stick to it or these little guys will get VERY confused. But, it is not too late for her to learn to go potty outside. It is just going to take a little longer bc she was first trained on the pad. I think what are you doing right now sounds fantastic.
At the rate you're going, she will definately get the hang of it and be potty trained soon. It is DEFINATELY not too late for her to learn. She is still very young.
I agree with what someone said earlier, when you see her going potty inside, clap your hands really loudly and say NO really loudly and sternly. I know a lot of people disagree with this, but what I had to do with my stubborn yorkie is when I saw him going potty inside, not only did I clap really loudly and say NO, but I also popped him on the butt with an empty paper towel roll. It doesn't hurt, it just scares them, kinda like the same effect as clapping. Then, after you clap and/or pop her with the empty paper towel roll take her outside where hopefully she'll finish up...then praise praise. I would not recommend the paper towel roll for puppies bc they really are too young and really just don't know any better. But at 9 months, she is definately a candidate for the paper towel roll bc she should know right from wrong at this point. I only use the paper towel roll on my 9 month old...not my 9 week old.
The good thing about what you're doing is that you have her on a leash so you can watch her at all times. I would carry the paper towel roll around with you stuck in your pocket or pants or something. Keep doing what you are doing by keeping her on the leash when you are home. The reason it takes longer for some is because when their owners are home, they let the yorkie have full run of the house (don't keep them on a leash)...then the yorkie goes in another room and potties and the owner doesn't see it until hours later. By then it is too late to punish them.
So, sounds like you are doing EVERYTHING right...just keep being SO consistent. Don't give up! You are doing a great job. If you keep taking her out of the same door, hopefully she'll learn to tell you when she has to go outside. However, my sister's yorkie has never let her know when she has to go outside. So, she had to be put on a feeding schedule and my sister takes her outside at certain times during the day. It worked for her and she is completely potty trained. She had to use the empty paper towel roll method, too. It worked great for her and me. Good luck. Keep doing what you are doing...you are doing a GREAT job.