I don't know about the T-Mo branded S9, but the unlocked Samsung Galaxy S9/S9+ has a bug that can consume hundreds of megabytes of cell data per day. When WiFi activates, it notifies apps of the change but it doesn't turn off their LTE data connections. Applications that already have connections open will begin doing all the WiFi-only work while still on LTE. This means e-mail attachments download, apps update, cloud sync starts, and video apps pre-fetch content while they're still on LTE data. My phone is using over 1GB of cell data a week while on WiFi.
Moderators in the Samsung forums have said that cellular data must be manually turned off every time WiFi is used. It's shocking to hear such a suggestion, but they are pretending that it's normal. On a side note, the unlocked Galaxy S9 series is not receiving any promised updates.