Just duplicate your page... and re-size it!
That's right, until Google decides to release a major update to its free tool, reports won't be responsive.
But, here's the trick. Instead of focusing on making your actual report responsive, what about making a duplicate version designed for mobile (of the full report or just a subset of pages where necessary. Spoiler: if you're to implement changes to one version, you'll have to double the work to change the second version as well, definitively not a silver bullet solution)?
How are we going to proceed? Select the page you want to make responsive, and duplicate it (again, as a new page in the same report or directly in a new report).
In the top menu, select Page, Current Page Settings, and in the Style ribbon, set the Canvas Size Width to 390px (why not 400px? By experience, many mobile phones are still 397px wide!).