Look at my shortcut Cronios, which implements a shortcut scheduler. You would have to create a cron job and shortcut that polled every minute. If the motion sensor trips, it would — via IFTTT — write to some file on Dropbox that the shortcut can read. If the file content went from 0 to 1, the triggering shortcut would run (within a minute of the sensor tripping).
For examples of remote triggering, look also at my WatchCuts shortcut. It uses Cronios and Reminders to automatically run shortcuts on iPhones and iPads from any iCloud-connected device, Mac and Apple Watch included.