DayViewer is a PWA (which stands for Progressive Web App) it basically means that a mobile app can be created instantly by going to your browser and adding DayViewer to your home screen.
This method of installion bypasses the Appstore and Google Play store but works just like the apps.
The DayViewer design works similarly on both large and small screens and has been carefully designed to meet the requirements for various devices.
Depending on the browser and device used, please follow the setup instructions as appropriate. It would likely be easier to use the Chrome browser which is available on Android and iOS devices.