t-mo modem questions

I hate to break it to you but the gateway never changes the 192.168.12.1 gateway address. Not ever. The T-Mobile home internet mobile application is the T-Mobile way to manage the gateway. The Arcadyan and Sagemcon provide NO other means of admin management. The T-Mobile application is a real bad piece of work. If you have an iPhone best of luck getting beyond the "Lets get you started!" page. Seriously I do hope you have better luck than most. I have the iPhone 12 Pro and that application almost never works. There is another post, Anyone's Home Internet App not loading? which has feeble attempts to get the thing working. Sometimes it does, and sometimes not. If you have an old Android phone stashed in a drawer some place, even with no SIM card, you can try loading the mobile application on an Android phone and probably get it to actually present the login page and not be useless. 

The only other avenue I can recommend is to go watch Nater Tater YouTube videos he has made about each of the T-Mobile gateways and how to backdoor connect and do management through more creative measures. He has posted a backdoor scripting that allows the Arcadyan configuration in JSON format to be extracted, edited, and then sent back into the gateway with altered configuration parameters for doing what is a real pain to do via the mobile application when it does not want to play.

Nate also presented another script for the Sagemcon that allows the user to step through turning off the wireless radios so another router connected to the gateway does not have those channels engaged causing possible interference. 

The 4G LTE gateway and the Nokia gateway are the only ones that allow the web GUI interface for management via the admin interface. Those are simple to access with a browser and easy to manage. The Arcadyan and Sagemcon require using the T-Mobile home internet mobile application OR one of the backdoor access methods as outlined by Nate via his Nater Tater YouTube channel.

I wish I had a much better answer for you but sorry I don't. I have the same problems repeatedly with the T-Mobile home internet application on my T-Mobile purchased Apple iPhone 12 Pro. I believe their mobile application may be worse for behavior on the iOS 16.2 than before maybe. I can now seldom get it to stop presenting the Lets get you connected! page. It presents the page where you have the option to try to login but then the application can't seem to connect to the gateway EVEN THOUGH the phone is connected to the gateway wireless network. Some management application eh? I have had very limited success where if I bring up that app and then turn the wireless off on my phone and then wait 30 seconds or so and then turn the iPhone wireless back on the mobile application MIGHT connect to the gateway and present the login screen. Sorry, but as many times as I have tried and all the time I have wasted trying to use that application and figure out a solid way to get it to work I have not been able to determine a foolproof way to make it work. OK, most of the time on my old Pixel that has NO SIM card and barely holds a charge the application works most of the time. So, an Android might run the application and not have you pulling your hair out. 

OR Go check out Nater Tater’s YouTube videos for the backdoor access methods.