I'm going to quote our all-star technical documentation writer on this one:
We recommend avoiding the routing traces on the top layer of your board, directly underneath the E Series module if possible. Because the E Series module does not have an antenna on the board itself, it's fine to route traces on lower layers and have ground plane under the module outside of the underside pads. However, the bottom layer of the module is ground plane, so as long you avoid the several exposed pads on the bottom of the module you'll probably be OK if you route traces on the top layer of your base board. Since it's only protected by the two layers of solder resist from shorting to the ground plane on the bottom of the module, we don't recommend doing it.