OK, more complicated than I thought, so . . .
I have the original remote that came with the Echo, and the back piece holder is magnetized and clamps really hard to a metal fridge. The remote itself is not magnetized, but is held by the holder's magnet. In case the back piece was not stuck to metal, there was a piece of (sticky tape?) included so it could be mounted on non metal and the holder magnet still held the remote. So, the only magnet is in the holder. If it was unmounted on non metal or not on metal for the whole time, magnets can lose their magnetism. So, unless you want to ask Amazon to replace the back piece, then use Velcro. because I do not believe the magnetic tape is a strong enough magnet to pass through the holder to then hold the remote. In fact, I would not use the holder, just put Velcro on the back of remote (fuzzy side) and the barbed side on what will be holding it.
Personally, I think it is worth a shot to ask Amazon to replace the faulty magnetic piece.