If the T-Mobile SIM card that you purchased was part of a T-Mobile Type Allocation Code (TAC) or International Mobile Equipment Identity (IMEI) restricted plan that was associated with a specific device such as the Samsung S21, then your SIM card is restricted to only work with the associated device.
To ensure that your T-Mobile SIM card works with your NETGEAR mobile WiFi device, it must be associated with T-Mobile's Bring your own device (BYOD) mobile data plan.
For more information, see: