Hi, and welcome to the forum. What is the longest she is actually left for? If she wants to go while you're there, does she actively ask to be taken outside? Remember that most people manage without pee pads so it is
workable to build in enough outdoor toilet breaks - but I can appreciate that it is more convenient, particularly when you are several floors up.
One thought that occurred to me is if you gave her something like chicken broth, that may encourage her to drink enough that she NEEDS to go while you are there so you can then encourage her to go on the pee pad. Don't do this before leaving her though as if she's decided she shouldn't pee inside, she could get really distressed and also hold on long enough to cause actual damage. Also, I don't know if there's a danger of such small dogs drinking too much - something to research first.
Another thought is to make inside more like outside. Work out what surface she likes to go on outdoors, get a large tray, and replicate it inside. It wants to be as realistic as possible, so actual soil/turf etc. dug up from where she normally goes. There are commercial versions, like this: https://www.amazon.co.uk/Indoor-Toilet- ... B00ZFLMS5O
But I suspect to a dog, that's going to look and smell more like carpet than grass. I think I'd combine the approaches - sort out the 'indoor toilet', make sure she drinks plenty, then wait for nature to take its course, encourage her to use the tray, and praise like mad if she does.