While this configuration setting does work on most Zebra devices I am not sure it will work on the TC21. The TC21 is a value tier device from Zebra that has been purposefully limited in functionality to keep costs low. This results in a dramatically reduced set of MX configurations for this class of device compared with the enterprise class devices from Zebra. This is frequently a rude awakening for end customers who buy the cheaper TC21 thinking its just like other Zebra devices, only to find out that it is severely limited from a configurability perspective. Based on the documentation you should be able to disable MAC randomization through configuration of the TC53, but the TC21 is not listed. Also note that MAC filtering has somewhat gone by the wayside in modern networks as there is increasing restrictions on device identifiers like MAC. Thankfully Zebra has built and exposed a configuration parameter to disable MAC randomization but the fact that newer versions of Android have this feature enabled by default now should be an indicator that MAC filtering on the network is no longer a reliable approach.