The attached uses a script from http://detectmobilebrowsers.com/ and will automatically, on load of the page, detect if a mobile (not tablet) is being used. You can alter the script to include tablets as well.
see https://stackoverflow.com/questions/11381673/detecting-a-mobile-browser) if you need that script.
I added a Lectora variable browtype (browserType is a Lectora reserved variable and fairly useless imo) and populate it with “true” for mobile, “false” for anything else. Seems to work well in my tests.