Default SSID still being broadcast.

I will assume you have the Arcadyan KVD21 T-Mobile gateway/router since you just set things up. There is a good YouTube video by Nater Tater which covers how to disable &/or make changes to the configuration of the KVD21 gateway. You should check it out. He provides the commands to obtain the text based configuration and how to edit it and send it back into the gateway. If you do not see the “TMOBILE-8888” SSID in the mobile application or the web view from 192.168.12.1 via a browser on the local network then you could copy out the configuration, edit out the “TMOBILE-8888” and related parameters from the config and then send in the edited configuration which should remove that. If you only want to turn off the SSID then just changing the true to false for the enable of that SSID will shut it down in effect. Yes, it would be a security hole but only from the WIFI side so someone local would have to access from the WIFI network.

You could try setting it to factory defaults and setting it up again. I would suggest first to web into the 192.168.12.1 interface and check the existing SSIDs you can see. It may have been when you made the change to one of the radios the save was not actually done or did not take. Double check the SSIDs that are visible in the web GUI interface then decide. Just for convenience sake I have attached a copy of the user guide for the Arcadyan KVD21.