@Kelsey3
If you have the Express 4k+, then you have model 3941.
For Roku devices, the compatible protocols are b/g/n/ac.
If you can change the ax mode to ac, you should be able to connect to the 5ghz band of the network. Basically you need to login to the router and [Edit] the 5ghz band so you can add "ac" to the Wireless Mode.
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For Xfinity issues the normal troubleshooting includes...
1. Edit the 2.4ghz band so Wireless Mode is set to b/g/n
2. Edit the 5ghz band so Wireless Mode is set to n/AC/AX (Roku cannot currently use AX)
3. Change/edit the wifi password to remove any special characters
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If you need a temporary workaround while working on the above settings, you can connect your phone or laptop to the Xfinity wifi, and then broadcast a "hotspot" so that your Roku device can connect to it and share the wifi. This method is generally called an ad-hoc hotspot. (you are using the phone/laptop "hotspot" feature to connect the devices, but actually are using your wifi so you do not use your hotspot/mobile data).
https://community.roku.com/t5/Roku-mobile-app/Re-Want-to-use-Roku-app-on-Roku-tcl-tv-while-it-is-con... (for Android devices)
https://support.microsoft.com/en-us/windows/use-your-windows-pc-as-a-mobile-hotspot-c89b0fad-72d5-41... (for Windows laptops)
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102