What is the model number of the router of the router you currently have? Look for sticker on side or bottom.
Some of the older routers are still configurable by user. The newer routers will almost certainly have b protocol disabled in their firmware.
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As for your 5ghz settings, they look fine as you reported ch. 36, n/AC. You can certainly try another compatible channel, but don:t think that is issue.
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Try going into router settings and either changing the SSID password to something very easy (no special characters) or leave the password empty (creates open network). Let's rule out a password issue with this step.
Change the password for both the 2.4ghz and 5ghz bands. (Or leave blank while troubleshooting). Then Restart the router. Then go to your Roku device and Reset Network Connection under Settings/System/Advanced System Settings/Network Connection Reset/Reset Connection.
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Personally, I would purchase my own router and tell Comcast/Xfinity to jump in a lake. Especially if you are leasing a router from them and they are refusing to configure it to work with the devices on your network.
I'll go in and change the password to something simple and let you know.
