Hello, mattybme!
Thanks for using Apple Support Communities, it's great to have you here.
One of the great features of using a Dual SIM setup on your iPhone is that you can use a local regional cellular plan when traveling abroad. We want to help get that all set up so you can enjoy this feature to its fullest extent.
To start, this resource has some really good information: Using Dual SIM with two nano-SIM cards.
We want to recommend that you work through those steps to set up your Dual SIM. You will also want to reach out to your carriers to see if they have some more information on setting this up with their specific networks.
We hope that this information is helpful.
Have a great day!