Responsive scaling ignored on mobile

I am using Popup Builder Free version to display answers for a series of topics: https://taastrategies.com/home/insights/

I want the content (text) popup to open entirely within the boundaries of the primary host page with popup scroll bars if needed and close button visible at lower right.

I am testing using Chrome browser with a Windows 10 laptop (1920×1080), Android Nexus 5x (1080×1920) and Nexus 10 (2560×1600)

I am getting exactly the behavior I want on laptop, regardless of browser window sizing, with the following:

Dimensions settings:
Custom mode
Width 95%
Height 95%
Initial width 300px
Initial height 100px
Max Width: empty
Max height: empty

Options: (checked)
Enable content scrolling
Enable reposition
Enable scaling

On laptop, the popup opens scaled at 95% width and height, popup scroll bars if needed, and close button at lower right. Perfect!

On Android tablet and phone, the scaling spec is ignored. The popup opens w/o scaling, with width approximately 80% of screen width, whatever length is required to hold the content, and the entire screen must be scrolled to reach the bottom of the content. This causes the reader to lose place within the series of topics.

If I put it in Responsive mode at 90%:

the laptop width is 90%; and popup scrollbars are enabled, however long content extends well below the bottom of screen requiring both popup and primary window scrolling. The tablet and phone do the same.

If I put it in Custom mode at 800px X 600px, I get 800px width but long content again extends well below the bottom of the screen on all devices.

Disabling popups on mobile is not a solution. What do I need to do to obtain the same desired behavior on all devices?