razz, as bae inferred, whoever told you that is confused -- or confusing.
DW has a cellular-enabled iPad mini. There are some options for data service which are less expensive than a regular cellular-phone plan with data. Hers has an AT&T plan that provides 1 GB of data over 90 days for, I think, $25. It can be renewed or discontinued at any time. It's not the cheapest data around, but DW lives with it comfortably and "fixing" that expense is far down my to-do list.
iPads are sold "unlocked" -- the device is not tied to a particular carrier, the way phones often are. We could pop a T-Mobile SIM into DW's iPad tomorrow and be just fine. There are some technical issues which muddy this a bit, but for your purposes, consider them unlocked.
I do know that there are less expensive cellular-data providers than AT&T. The phone service reseller H2O, IIRC, is one. T-Mobile had a deal in which you got a SIM from them and you got 200 MB a month for free; you didn't even have to be a T-Mo customer. Some carriers may let you add the tablet to the "bucket o' data" you share on other devices on your plan. The best way to find out what kind of deal you can get is to ask your cellular provider.
Others suggest that using your smartphone as a hotspot will work, and it will. It's a bit putzy to set up, though, and some plans (primarily prepaid/MVNOs) do not allow hotspots. If you are the kind of person who will resell your tablet sometime in the next couple of years (to upgrade or whatever), tablets with cellular-data capability do fetch more on the market. If we had to replace DW's iPad tomorrow, we'd buy another cell-data-enabled one. It's convenient and it's not anything you can add after the purchase. And we can keep an eye on the data enough to not be surprised by a huge bill. It's a nice "add".