Hi @NicoleDIY,
As per the STRUCTAflor Installation & Design Manual, it is always recommended that you use a fibre cement underlay between your tiles and the Yellowtongue. This is because Yellowtongue is susceptible to expansion and contraction, and if tiles are applied directly to it, this can crack the adhesive and grout. While it may have been what was done before, it is not recommended where it can be avoided.
The issue in your situation is that this would add 6mm to the height of the floor, and as yours has already been installed to match the height of the existing flooring, it would mean there would be considerable work elsewhere to ensure things are level.
I imagine you have had some issues due to water coming in through the door?
Ultimately, what you have done by applying a waterproofer to the Yellowtongue is about the best thing you can do right now. If you were to run into issues in the future, though, I would suggest you do not use Yellowtongue, instead, I would suggest you use a fibre cement flooring product like Secura Flooring, which is not susceptible to rot and would be fine to tile directly over.
Let me know if you have further questions.
Jcob
