You didnt mention your Roku's model # and firmware version(s): Settings/System/About
1) Try rebooting all devices (modem/router/gateway/Rokus)
2) Verify your modem/router/gateway has its 2.4Ghz SSID enabled, and on channel 1, 6, or 11 with a 20Mhz width. For the 5Ghz SSID, make sure its on channel 36-48 or 149-165. Recent firmware updates for provider equipment have disabled 2.4Ghz, so make sure its enabled.