how do i use my device dual sim

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Using Dual SIM with an eSIM https://support.apple.com/en-us/HT209044

Using Dual SIM with two nano-SIM cards https://support.apple.com/en-us/HT209086

>Using Dual Sim with eSIM:

An eSIM is a digital SIM that allows you to activate a cellular plan from your carrier without having to use a physical nano-SIM. With iPhone 13 Pro Max, iPhone 13 Pro, iPhone 13, and iPhone 13 mini, you can use Dual SIM with either two active eSIMs or a nano-SIM and an eSIM. iPhone 12 models, iPhone 11 models, iPhone XS, iPhone XS Max, and iPhone XR, feature Dual SIM with a nano-SIM and an eSIM

To use two different carriers, your iPhone must be unlocked. Otherwise, both plans must be from the same carrier. If a CDMA carrier provides your first SIM, your second SIM won't support CDMA. Contact your carrier for more information. 

>Using Dual SIM with two nano-SIM cards:

In China mainland, Hong Kong, and Macao, certain iPhone models feature Dual SIM with two nano-SIM cards. This gives you two phone numbers to make and receive calls and text messages.

The resources above will guide you through the setup and usage of dual SIM

Hope this helps