If you are an XFinity user, try using the XFinity mobile app to change your 2.4Ghz SSID settings to channel 1, 6, or 11 with 20Mhz bandwidth and the mode to "B/G/N":
"I saw this 'Note: Roku Express and Roku Premiere/Premiere+ (3920/3921) models support only 802.11 wireless b/g/n networks (the 2.4GHz band).
I checked my XFi app for my router settings and it was not set to b/g/n , it was set to g/n. When I changed it, I was able to connect to wifi, no problem.
Below is where I found the settings in the Xfi app.
Go to the Connect tab. Select See Network. Select Advanced Settings. Select 2.4 and 5 GHz WiFi."