The answer to your question probably lies in your own CSS skills. I'm not quite sure what you mean when you say "the portrait monitors are larger than that size and thus do not trigger the responsiveness." What is "that size" and how did you determine this dimension?
No matter what "this size" means, This suggests that your CSS media queries may not be set for your specific needs, which appear to be unique to your environment. CSS media queries in a commercial product like Formidable will never be able to meet all specific needs out of the box. Formidable functions in a BIG world and the number of use cases it will encounter in real life are enormous. This is why developers code for common ground issues.
As for your need, do you know how to use your browser's inspection tool? If not, look it up in your browser help. The inspection will help you pinpoint the CSS classes that affect your screen width. You can test new CSS values and adjust the screen width to your liking. When everything meets your needs, copy the classes and new values to the Formidable Custom CSS area.
If you need help, you can find highly competent Formidable Developers in the Formidable Masterminds Developers Directory. https://formidable-masterminds/developers-directory/.