iPads purchased through a Cellular Carrier may be subsidy locked - AT&T being one carrier that is known to routinely lock devices to its Cellular network. You may be able to have the subsidy lock removed; where possible, you may be charged. Also be aware that the built-in eSIM of some models of iPad may also have been permanently disabled.
Be aware that Apple cannot (or will not) remove a subsidy lock from a device. Also be wary of third-party “unlocking services” that claim to be able to remove the lock; most are unreliable. A subsidy lock can only be removed by the carrier that applied the lock.
When purchasing a WiFi+Cellular model of iPad, despite seemingly attractive deals offered by Cellular Carriers, purchasing directly from Apple or Apple Authorised Retailer largely removes the risk of encountering a subsidy lock when changing Cellular provider.