I don’t really know what your issue is but offer a suggestion. The Sim tray in all these watches are very similar to the one pictured here. On the top of the Sim tray is the cover that is marked “Open” and “Lock.”
You slide it in the direction shown by the open arrow (on the right side.) insert the Nano Sim Card correctly so it is nearly flush and then slide the cover back over the Sim card to the “Lock” position, as shown, to secure it in place.
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