If the ‘fake’ grill is made of plastic rather than metal, WiFi gear can be mounted behind the grill.

Mounting an air grill up high on the wall is not uncommon in areas with air conditioning. Paint to match the wall and it can be near invisible. Louvers angled upwards for high mounted grills.

If the ceiling is not large, one way to run cables is to strap the ceiling with wood or metal strips and install a second layer of drywall below the original. Run your wires between the two layers. Tape and finish the new ceiling drywall where it meets the walls.