Have you tried using Samsung switch on a pc? If there is an update that is the quickest way to get it. T-mobile OTA updates may take a bit longer to get to you
Hey, georatm! I don't blame you for wanting Nougat for your Galaxy Tabs. I don't know for certain if that's something that Samsung will decide to develop, but you can follow the Software Updates page for emails when new devices are added to the list. There were updates for the S2, which was released only a few months after the Tab E, but I can't say if they're working their way backwards through the roster. :/ Fingers crossed, though!