If the deck is attached to the house, they should be able to plug a pod on or near the exterior wall. If the deck is long, they could even use 2 pods at opposite ends of the deck.
I looked up the info on the Plume site and found the following, which means the in-use cover will probably trap too much heat. They could keep that idea as a last resort if nothing else works. A cover with a larger opening at the bottom is probably better for heat dissipation, keeping in mind there will be at least a small opening for the electrical cord, which should be kept open.
Temperature: 32 - 95 F (0 - 35C)
Humidity: 10% - 90% relative
More info: Can I plug a pod outdoors?
We recommend that you keep your Plume pods inside and away from rain or moisture. If you want to improve Wi-Fi coverage in your outdoor space, we suggest that you place a pod just inside your door or take it outside only when you need it for a get together.
They may need additional pods between the exterior wall and the router to relay the signal. From the Bell MTS FAQ:
Once moved, pods require a minimum of 10 minutes to recalibrate their connection to the rest of the network. Allow for up to 48 hours for full optimization.
They can rent them or a month for $5 to see if it works well in their situation.