Modal problem on mobile

Hi there,
I am at beginning and I apologise for my incompetence (I’m not a programmer nor I know the script languages) and for my english (google translate!).
I am working with Blocs v3.4.4

I have added a Modal and it is working but, on the mobile version, it create a problem, it is like if the image is bigger than necessary so that on mobile version it’s possible to see only the transparent outline area but not the modal and it is impossible to close it. This is possible to see in the preview mode as well as online.
I have tried to switch off the bloc of the modal for the mobile version ad it disappear from the mobile version but now there is a big white area like if a white bloc is switched on.
I have tried to change the measures of the modal but I am not capable to do (it changes on back-end but in preview and online it is the same) and I have tried to add a new “smaller” modal to show only on the mobile version but I am not capable to do and I don’t know how to make appear two modal at the same time one on pc version and one on mobile version.

Thank you in advance for the help.