Run device manager and check the WIFI card's settings to see that 802.11n is available. You may have other Toshiba/Realtek tools installed that can also check the settings.
On a working machine, go to the Microsoft Store and install Matt Hafner's free WiFi Analyzer program. Select the Networks icon and then in the lower right corner of the window select "show more".
Find your 2.4ghz network and see if it reports 802.11n. If it reports something else like 802.11ac, then see if you can match up the NIC settings to the broadcast protocol.
If you still can't get it to connect, your best bet is to call T-Mobile support.