In addition to native Android and iOS app. There is a universal version that can read books sent to the cloud supporting all common operating systems (Windows, Linux, Mac. In addition it can be installed also on Android and iOS so you can use this universal app instead of the native one on Android or iOS. It does have some disadvantages though so keep reading.
Using chrome (or Safari on iOS) go to: https://pwa.heliconbooks.com few seconds later you should see a dialog asking if you want to install the app. On Android and other os's you simply click install and confirm and the app. will be installed.
On iOS you have to click the share menu and then "Add to home screen".
iOS share menu looks like this:
If you don't see the install dialog, in chrome there is a circled + sign at the right side of the url line, click it and confirm to install. In Android choose 'Add to home screen' from the chrome menu.