To do that, go to: settings > Mobile Data > Mobile Data again, select Secondary: the tick will appear in front of it when it is selected 
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Important: When you have multiple SIM cards/eSIM profiles on your iPhone, it is not possible to activate cellular data on two data plans at the same time. You can only apply it to one at a time.
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How to check eSIM compatibilitySmartphones / Tablets:
Dial *#06#.
→ If an EID number appears, your device is eSIM-compatible.
Windows 10 / 11:
Press Windows+I.
Go to “Network & Internet”
→ If you see "Cellular", your PC is eSIM-compatible.
How to check carrier unlockingiPhone / iPad:
Go to Settings > General > About.
Scroll to Carrier Lock.
→ If you see “No SIM restrictions”, your device is unlocked.
Android:
Go to Settings > Network & Internet > Mobile network.
Disable Automatic network selection.
→ If multiple networks appear, your device is unlocked.
