I’ve recently purchased an iPad Air advertised as ‘Wifi Only’ but it has the sim card tray & antenna. I’ve got the exact same model A1475 in a different storage capacity variant side-by-side with it, the model numbers match. Both are indeed Wifi + Cellular models.
Using Apple's site https://checkcoverage.apple.com and entering the serial number to check the service and support coverage of the devices, both are shown to be 'iPad Air WiFi, Cellular'
The serial numbers on the back of the devices match the serial number in ‘Settings->General->About’ for each device respectively.
The one lacking cellular capability also lacks the following fields in the ‘About’ section: Network, Carrier, Cellular Data Number, IMEI, ICCID, MEID, Modem Firmware. It does not have ‘Cellular Data’ or ‘Carrier’ in the first category of settings either as the older iPad does either, usually 'Airplane Mode, Wi-Fi, Bluetooth, Cellular Data, Carrier'.
I've tried 2 different sim cards, both which work in my phone and other cellular iPad but there's absolutely no indication that it's been inserted or lack thereof.
Other suggestion around the web and on the Apple forums indicate to restore the iPad, users have reported only WiFi firmware is compatible and resetting network settings (I've reset the entire device), all to no avail for other users either.
Browsing other forums I’ve learnt that:
The software setting up the antenna may have failed during a software update therefore no longer has the cellular capability. (iPhones are not prone to this however)
The board within the device or certain components could have been removed in order to remove an iCloud lock that existed on the device.
I picked up both second-hand for dirt-cheap prices and the newer iPad was originally advertised as ‘WiFi only’ anyway so there’s no issue with that, I would just like to find a way to resolve the issue is possible. I’d be grateful for any suggestions.
Edit 01/06/2017 I opened up the iPad and found that a cable from the sim card tray to the motherboard where the screen ICs are connected was unplugged. When turning the iPad on, it attempts to look for a signal (shows the 5 empty circles) but then changes to the text 'iPad' but without the pop-up that reads 'No sim card inserted'. I may have damaged the cable when inserting it, it was a mission to get in, that's probably why the last person who repaired it didn't put it into place.