You didnt mention your Roku model # and firmware version: Settings/System/About
If its an Express, its a 2.4Ghz only model - Try manually configuring (in your modem/router/gateway) the 2.4Ghz SSID for channel 1, 6, or 11, with 20Mhz bandwidth, and "B/G/N" mode (especially if XFinity is your ISP).