Microsoft have made a change on mobile devices where you now need to install the CoPilot app in order to access Office 365, you may notice when you sign in that you see the screen below asking you to install this.
If you go to the Play store/ app store on your phone and search for Copilot you'll be able to install it there.
Once you've installed the app you'll be able to access Office 365 when you refresh the page.