You can study on your smartphone or tablet by Reallyenglish app.
When you search with a keyword "Reallyenglish" on App Store / Google Play, you will find Reallyenglish app (silver icon with "R" character).
Please download it and sign in with your registered email.
For the detail of the mobile app, see here.
Please be sure that some of the courses are not supported for the app use although you can study on the mobile browser.