If Dolby Atmos is not showing up as an option under Spatial Sound in Windows 10, there are a few things you can try:
Check that your audio device and drivers support Dolby Atmos. You mentioned that your LG B2 TV and Sonos Beam soundbar support Dolby Atmos, but it's also important to make sure that your audio drivers are up-to-date and support Dolby Atmos. You can check for driver updates using the Device Manager in Windows.
Make sure that Spatial Sound is enabled in Windows. To do this, go to Settings > System > Sound > Sound Control Panel > Playback tab, select your audio device, and click Properties. Under the Spatial sound tab, select "Windows Sonic for Headphones" or "Dolby Atmos for Headphones" if available.
Try disabling other audio enhancements. If you have other audio enhancements enabled, they may interfere with Dolby Atmos. To disable other audio enhancements, go to the Enhancements tab in the Properties window for your audio device and uncheck all enhancements.
Check that Dolby Atmos is enabled for your audio device. To do this, open the Dolby Access app and make sure that Dolby Atmos for Headphones is enabled for your audio device.
Try resetting the Windows audio settings. To do this, go to Settings > System > Sound > Reset, and click the Reset button under the Reset your audio settings section.
Try reinstalling the audio drivers. If none of the above steps work, try uninstalling and reinstalling the audio drivers for your audio device.