I've been doing on-line lessons for my students for some time. Although I use a PSR-S970 for teaching, I'm familiar with the PSR-S900 as I had one for some years. Some of my students have them.
I can teach anything from the basics to advanced keyboard - arranging, making the keyboard really work for you etc - right up to Diploma level. Several of my students now have letters after their names! And I can build music theory, bass clef reading, scales etc into the process. If you want to try exams, these are also now all available on line, and are great fun.
If I can help, just message me or email me via my website, which you can look at to find out a bit more about me!