A US Cellular G4 won't have T-Mobile WiFi calling built into it. There is no 6.0.1 update either.
With only a handful of exceptions, WiFi calling is unavailable on devices that are not T-Mobile branded.
You can attempt to download the latest firmware from LG. I believe the LG Support Tool will check for any applicable firmware upgrade.
The support tool will not find an update. 6.0.1 is not available for the Leon. Also, the firmware will not install wifi calling.