I have this model of Belkin router
runtime Code version F5D7633-4Av1_UK_1.00.17
which powers my main slug setup. Firefly on slug shows up perfectly on a wireless Mac in iTunes. Equally I don’t see any multicast option in the router menus, but first up try switching to a different channel from the default.
Then, if possible or something similar:
Under Wireless> Channel & SSID
have: 54g-auto enabled 54g protection ON Turbo Mode – Frame-bursting Mode
If there is some other mixed-mode option, check it as your Roku is a b-mode device.
you don’t mention the Roku’s location relative to the router. Bear in mind walls, distance & other interference/environmental factors which may have a bearing on things…