The carrier has nothing to do with the updates getting pushed to a phone. That said, they DO approve updates for devices branded to them. In other words, if you purchase a device that was originally an AT&T phone, but was carrier unlocked, it's still technically an AT&T phone, so AT&T tests and approves any updates, and they get pushed out by the manufacturer via the "check for updates" button (which I feel is a con based on the fact it's never worked for me).
If the device never belonged to a carrier (generally meaning international devices because the US doesn't tend to sell unlocked devices... Google Nexus / Pixel being an exception, and iPhone being a partial exception with their BS that they do), then the update comes from the manufacturer after their tests have been done.
All the above assumes you're running a FACTORY STOCK ROM. If your device was modified in some way (maybe it was an international device that had T-Mobile's ROM installed on it? Or an ATT device that was unlocked and had a T-Mobile ROM put on it, or even a device that has another non-factory ROM on it), check for updates will not work.
Based on the fact it's April and your last security update was December, I'm imagining the possibility of your device not being stock. If it's not stock, the forums here cannot help you. If it's not a T-Mobile device, the forums here cannot directly help you, only tell you to look to the carrier the device was designed for to see if they've released updates.
LG V30/V30+: Update Complete says there's currently no Oreo update for the device from T-Mobile.
Software updates: LG V30/V30+ shows the software version for the most recent update T-Mobile has available for their device. If the build number matches somewhere in here, it IS a T-Mobile device. If it doesn't, it's not a T-Mobile device.
It's here, got it last night finally, with April security update.