To update mobile image and enable mobile view banner, follow the below steps.
Step 1: Navigate to Mageplus > Banner Sliders > Manage Banners
Step 2: Look for the record you want to edit and click Edit link
Step 3: Scroll to Upload Mobile Image, click Choose File button then select your new portrait banner. Once done click Save Banner button to apply the changes.
NOTE: Please make sure all the mobile image is in same width and height otherwise on website will not show the consistent height on the banner when it slide.
Step 4: Navigate to Mageplus > Banner Sliders > Manage Sliders
Step 5: Click Edit link on the homepage slider record to edit
Step 6: Change the Type setting to Desktop and Mobile then click Save Slider button to apply the setting
Please ensure the mobile image is show portrait size image. Below is the sample from staging site.
If on website didn't reflect the changes, please Flush Magento cache and view again.