iPad SIM is not supported due to USA country lock

You’ll need to contact the original “subsidizing” carrier who locked the phone and request that it be unlocked.

(Although nothing necessarily compels them to do so)

Apple can not bypass this feature or unlock it for you.

See:

How to unlock your iPhone for use with a different carrier - Apple Support

The iPad itself isn’t disabled - but if subsidy locked, Cellular connection will only be available for the network to which it is locked.

Devices purchased with a Cellular contract are very often paid-for over the life of the contract - instead of paying “up-front”. So as to ensure that devices are fully paid-for, the Cellular operator will hold-you-hostage - only releasing the subsidy lock when your minimum contractual obligations have been fulfilled (time - or early termination of the contract by “paying-off” outstanding fees).

If you have acquired a previously used device, that it subsidy locked to a Cellular Carrier that is not represented in your country, you may be unable to have the subsidy lock removed. Only the Carrier to whom the device is subsidy locked can remove the restriction; Apple will not be able to assist.

IdrisSeabright wrote:

To the best of my knowledge, iPads are never carrier locked.

iPads purchased from Apple are unlocked as you state. That said, when purchased from a Cellular Carrier, they can (and often are) subsidy locked - as US Customers of AT&T have reportedly often discovered.

To the best of my knowledge, iPads are never carrier locked.

They never were when I worked in the U.S. cellular industry. However, I left in 2016 so, perhaps that's changed.