In fact you don't have to assign dial-plan manually like what you did above. When u assign a license to a user, you have to choose "a location" for the user, just select respective locations for users.
For example, for china user, choose "China" and for Singapore users, choose "Singapore" as their locations, the Dial-plan will be automatically assigned to the users.
You don't have to do anything extra.
If you want to check which dial plan a user is associated with, just run this command on the PowerShell.
Get-CsEffectiveTenantDialPlan -Identity amos.marble@contoso.com
Ref: https://support.office.com/en-us/article/Create-and-manage-dial-plans-7af17c94-5f8f-4452-ae1d-01f495b4dc94