iPad Air 3 (128GB) won't update past iOS 12

My iPad Air 3 is stuck at iOS 12 while it’s compatible with iOS 18. It says it’s up to date in the settings. On iTunes, it says I can update it to iOS 18 but when I click on update it doesn’t work, even in DFU and recovery mode. It reboots and doesn’t work. It’s a 128gb second-hand iPad Air 3rd gen, the weird thing is that Apple didn’t officially sell a 128gb iPad Air 3, they only sold 64gb and 256gb, so I think my iPad has been modified with a custom hard disk or something. I also found out after removing the case that the aluminum back has some holes in it covered by stickers, rectangle holes where you can see the internal components.

[Re-Titled by Moderator]