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Please try below instructions one by one and see if it working :-
Ensure your SmartGlass device and console are on the same local network.
Check that your console is set to allow SmartGlass connections
On your console go to Settings->Preferences->SmartGlass Connections
Ensure it is set to “From any SmartGlass device” or “Only from profiles signed in on this xbox”. If set to the latter then you will need to ensure you are using the same profile on your SmartGlass device that you are signed in as on the console.
If you modified the settings then attempt to connect using SmartGlass
Restart your router
Unplug your router for 30 seconds and plug it back in.
Once you regain network connection on your console and device attempt to connect using SmartGlass.
Perform a full reboot of your console.
Press and hold the white Xbox button on the console until it turns off, or pull your power cord.
Wait 30 seconds to a minute and turn your console back on.
Once the console is back at the dashboard attempt to connect using SmartGlass
Common networking issues:
Xbox and device are on different subnets.
Symptom: Console discovery will not work.
Work around: You should be able to use the IP address of the console to connect despite being on different subnets. On your console go to Settings->Network->Advanced Settings to obtain the IP address of the console.
Resolution: Inspect your network setup\settings. Ensure your router is set to give out IP addresses in the same subnet regardless of whether you are wired or wireless. If you have more than 1 router make sure that only one of them is setup as DHCP.
Some wireless routers have a feature called "wireless isolation" mode, "guest mode" or something similar.
Symptom: Devices and console have internet access but devices cannot discover the console nor will using the IP address of the console work.
Resolution: Inspect your router settings and turn off isolation mode if it is enabled. Ensure neither your device nor console is on the guest network.
Some Dual-band wireless routers allow users to specify the same SSID for both the 2.4ghz and the 5ghz bands.
Symptom: Devices and console may appear to have internet access but devices fail to connect to the console.
Resolution: Inspect your router settings and ensure that you have different SSID names. This can simply mean adding a “5” at the end of the SSID name for the 5ghz range. Ensure that your devices connect to the proper SSID.
Please let me know, Did this helped ?