Windows 10 can't set connection to metered

You can use netsh wlan set profileparameter name="Your profile name" cost=Fixed in an elevated promt to change your connection to metered and netsh wlan set profileparameter name="Your profile name" cost=Unrestricted to change it back.

The name of the profile is usually your wifi SSID - you can check this using netsh wlan show profiles and check your current settings using netsh wlan show profile name="Your profile name".

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Shamelessly stolen from this answer.

You need to log in with an administrator account in order to set a WiFi connection to metered.

Note that when you are logged in as a standard user, not only can you not change the setting, but you can't see the setting either; the settings panel will show the "Metered connection" option as off regardless of what the actual setting is. Note that this setting is global, not per-user.

A standard user can check the setting from the command line:

netsh wlan show profile name="whatever"

Look under "Cost Settings" and if the Metered connection setting is turned on, the Cost will be set to Fixed, meaning "The use of this connection is unrestricted up to a specific limit." (See NetworkCostType Enum on MSDN.)

If the Metered connection setting is turned off, the Cost will be set to Unrestricted, "The connection is unlimited and has unrestricted usage charges and capacity constraints."

There is also a third option, Variable, "The connection is costed on a per-byte basis." However, I'm not aware of any way to choose that from the GUI.