How do you know that the device is connected to the correct network?   What if all network connections had been disabled by device end-user before the device got powered off/soft-reset?  In such case, no connection will be available unless the device can be successfully rebooted & logged-on (say, with a correct locksceen password), and device connections re-enabled explicitly via Settings options  by the device end-user.

Regarding whether the script command can reset your Zebra TC21 password, you can do tests for different scenarios (say send script when device is powered on/off, sent with MC agent/platform notification service, etc.) yourself.   If you have on-hand another Zebra TC21 running the same OEM firmware, you can try to send the same script command and confirm if it works or not on the device.