Pop Up Pro pop-ups not working on iphone - special settings?

Darn! Client was testing website on iphone and we now realize that the PopUP Pro successfully forces a grey screen, but the ads DO NOT show up.

I realize now that I checked custom size 600 x 600 for desktop experience.
I need the pop ups to be responsive.

Is there another way (via CSS on each ad) to assign the pop ups for desktop
to be size X and keep the default settings for mobile?

Need help!