The Volas Mattress Topper listing does not mention straps or grippers. The method that matches a normal bed is correct size, centred placement and a fitted sheet over both layers.
A topper that is smaller than the mattress will drift. A larger one will hang. Use the exact listed size, then pull the sheet corners fully on.
If it still moves, remake the bed and check Twin versus Twin XL, or King versus California King. Do not stretch the fabric to invent a fit.
Choose the matching variant on the Volas Mattress Topper listing.