Over the lats six months or so I have had to debug a couple of interesting issues with iPads, in particular iPads with cellular on them. The two devices were an 2nd generation iPad and another was a 2nd Generation 12.9-inch iPad Pro. Let us look at both of these, starting with the 2nd generation iPad.
2nd Generation iPad
The issue that this iPad was exhibiting was that it would start randomly saying "Activating iPad". The issues as intermittent and would do this upon reboot as well as when waking the iPad up.
Rebooting it would not fix it, as it would still give the same issue. Sometimes, letting it sit and attempt to activate would allow it to work, at least for a while. Before it would just be intermittent and happen occasionally, but starting in May it would do it consistently.
While sitting and pondering it for a bit, I came to realization of what was happening. The 2nd generation iPad had a 3G modem in it. The reason that it started doing it consistently in March was because the T-Mobile 3G service began to be shut down in March.
The fix for this was quite simple, remove the 3G SIM card from the iPad, and it works perfectly fine now.
2nd Generation 12.9-inch iPad Pro
The second iPad that has been having issues is a 2nd Generation 12.9-inch iPad Pro from 2017. The issue started with the iOS 15.0.1 update. Updating to iOS 15 on this device was no problem, but starting with iOS 15.0.1 it would no longer update.
The update would download, but upon trying to install it, it would get stuck on "verifying update". No matter how long I let it sit there, it would not do anything. I tried updating using a Mac, through Finder, but that also did not work. The same thing would occur, it would just sit there and spin at the "Verifying Update" window. I could reboot the iPad, and try again, but it would not do anything differently, just the same results.
Now, being the Apple nerd that I am, the fact that it would not update bugged me to no end. So yesterday, I wanted to really figure out what the issue was. I had tried doing some searching online previously, but everything that came up would lead me to a bogus solution.
That is, until this thread (available at https://discussions.apple.com/thread/253245929) came up from the Apple discussions forums, why it did not appear the previous times I did my searching, I do not know.
The first solution suggested to "Insert a SIM card and then do the update", and after inserting a SIM card, and re-trying the update, it actually worked. The second most helpful suggestion on that thread indicated that simply removing the SIM card tray would fix the issue. I did not try this second solution, because the first one worked. It should be noted that the SIM does not need to be an active one, just any compatible SIM card would work.
Obviously, there must be an issue with iOS 15 and verifying an update on a cellular iPad Pro when there is no SIM card in the slot. It is not yet known if the same thing will happen when updating to iOS 16, or if Apple will actually fix it. It seems like these types of issues wait until major versions to be fixed. At least there is a workaround that does indeed work.