these software update notifications on my phone are from T-Mobile and it's not a T-Mobile
I did purchase it off EBay listed as refurbished.
What is the model number of this S7? I suspect it was originally from T-Mobile and that's why you are seeing problems with updates. You need to understand how system updates are delivered. Carrier branded handsets will get the updates through the carrier. If the handset was originally unbranded or international, then the updates would come from the manufacturer, Samsung in your case. Only the Pixel/Nexus phones (with a few very specific exceptions) get system updates directly from Google.
So, if your S7 was originally branded by T-Mobile, it may see a pending update, but if your carrier is not T-Mobile, it may never download and install it.
You have a few options. The first is to simply ignore the notification. Accept the fact that you may never get this, or any subsequent updates. I doubt the S7 is being actively updated anymore in any case. The second is to pop a T-Mobile SIM card in the phone and see if you can apply the update that way. Once updated you can replace your card it it should be fine. If, as I suspect, S7's are not getting updated anymore, you wouldn't get any more notifications.
Another option would be to connect the phone to Samsung's servers using Smart Switch and see if that will let you update the phone. Sometimes carriers will share their updates with manufacturers to take some of the load off of their update servers. If it works, it will update you to the latest T-Mobile OS released for that device which may not be the latest version of Android that was released for the S7. It all depends where T-Mobile stopped.
More risky options would be to try and flash an international ROM to that device. In many cases the branded and unbranded hardware is exactly the same except for some radio band support. T-Mobile used to use some funky bands, but lately I think their phones support most GSM bands. Or, you could install a custom OS like LineageOS, depending on your comfort level with modifying phones and your willingness to risk losing data and or the phone itself.
