Cannot find my cellular data account in ios12

I just went through the same thing. I can confirm that going to att.com/ipadlanding brings you to the account maintenance page. Use your email and password for AT&T like you used to. However, it seems the "shortcut" in the Cellular settings page is gone. I was just in an Apple store, and they didn't know either (but I was talking to a sales associate about buying an iPad Pro -- I didn't go to the tech people), and I went to the AT&T store and they had no idea what I was talking about either. The good news is that the account page and same plan options are still there -- you just can't get to it from Settings*. I think one other detail is that it used to enable the cellular for "free" to get to the at&t site, in other words, if you were in the middle of nowhere with no WiFi and realized that you needed cellular data, you could use AT&Ts cellular data to sign up again. It seems that no longer the case, since you need to "browse" to the AT&T account page. So if you know you need the data plan, make sure to sign up while you still have WiFi! Strangely I couldn't find any ios12 explananation for this change, but it does seem consistent with what others are reporting.

I gave you the article so you can get to your AT&T account settings another way such as using a web browser to login to your AT&T account. You can also try removing and reinserting the SIM card.

Additional steps

To check DataConnect Pass balance and expiration date:

1. Go to att.com/iPadlanding and sign in to your DataConnect Pass account.
2. From the Account Overview screen, locate Status to view data amount used and Billing Period to view expiration date.

[replying to my own post to add info -- not sure how to edit] By the way this seems like a bit of a "bug/oversight" in ios12. Because once you enable Cellular, if your plan is expired (as mine was), it still brings up the popup dialog to say "go to Settings to open your account" but if you press "OK" nothing happens, since that option is no longer there in Settings.

Have you tried a force restart by holding the power button and home button simultaneously until the device restarts? In the meantime, this article may help you check your AT&T account status:

https://www.att.com/esupport/article.html#!/wireless/KM1166085