PDF Form Validation

Hi folks,

I have prepared a PDF Form for our client's (a simplified option attached below) where they can pick a clothing product in one dropdown and it will updated the sizing dropdown with a relevant chart (S, M etc. for unisex and 10, 12 etc. for fitted). It works perfectly on desktop but the validation doesn't seem to kick in on mobile.

Also, would it be possible for the size dropdown to updated the chart before you click on it? Right now it only updates it after clicking into size.

Any ideas how I could fix this?

Thanks in advance