I've gone on a couple of service calls to troubleshoot poor Wi-Fi performance where the wireless router was sitting on the floor up against a metal filing cabinet. Simply moving the router up on a shelf away from large metal objects was the primary fix.
Having your device enclosed in a metal box would be the best way I can think of to discourage any Wi-Fi transmission at all!
Some routers have external antenna connections which can be a solution to problems like this, but most all-in-one gateways (like yours) don't have external antennas.
Checking your speeds from your Ethernet-connected computer will be the next step in determining if Wi-Fi is compromised.
If so, the device may have to be relocated outside of its Faraday cage or install a separate remote wireless access point (and disable the router's internal Wi-Fi).