Dashboard is web-based, it can be viewed from both phone and computer.
This will signal that regardless of device or platform, if you have a web browser then you can use this.
If you need additional puffery then device-agnostic could also be used but you may alienate some readers/listeners.
I do not condone the use of cross-platform nor multi-platform in this "dashboard" situation because those words tend to indicate that you have a Windows installer, a Mac installer, Android installer, iPhone installer, etc... for your dashboard.
Google Chrome is truly multi/cross-platform. Firefox is multi/cross-platform. Microsoft Word is multi/cross-platform. These programs have dedicated installers on different platforms.
The web pages which you visit in web browsers are precisely that, web pages.
By association, the web pages are multi-platform because they are viewed in a multi-platform software.
If you wanted to literally advertise your dashboard as multi/cross-platform then that would mean your back-end dashboard software can be installed on a Linux or Windows server and will work on PHP, ASP, Ruby, Python, JSP, and/or Perl and can use a variety of databases.