Unlock iPad Air 1st Gen cellular carrier

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 by the Carrier to whom the subsidy-lock related; 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.

Carrier SIMs should not be passed-on when you sell an iPad - as the SIM relates to your Cellular plan/contract. When disposing of your iPad, you should remove a physical SIM if fitted - and, if necessary, you should contact your carrier to cancel your cellular plan.

Remove or switch the SIM card in your iPad - Apple Support

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

are you sure it's sim locked? i've only heard about ipads being sim locked in japan

but if it really is then only verizon can unlock it

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support