Device settings issue

The devices are trying to hold on to the connection while being in a sleep state where the modem is disconnecting them after so long of inactivity.  

Thanks for letting me know "what's going on". Let me now phase my question more clearly: how to I fix it?

There is no fix.  You could run a Access Point off of the modem for WiFi to resolve it by connecting those devices to the AP.

syaoran wrote:

The devices are trying to hold on to the connection while being in a sleep state where the modem is disconnecting them after so long of inactivity.  

That can’t be right. The device I’m using right now shows ‘offline’. Everything is offline.

It might be a synch issue with the mobile application. It might just need time and to record activity from the clients to refresh the data. It may be related to the adapter settings. If the client sleeps maybe the adapter still stays somewhat awake and does not also go into a sleep state. You might need to examine the network adapter's settings. 
When the mobile application works on my iPhone 12 Pro I can see devices but it has been a while and I seldom fight with the mobile application as I have the Nokia gateway. Maybe simply restarting the T-Mobile home internet application will allow the refresh of the information. 

Our new 5g gateway has been running for 3 days, and all devices always show as offline. Even after restarting the gateway, everything is offline.

Which gateway do you have? The Sagemcon I am guessing. Does the LED display on the front of the gateway have the bars displayed or is there NO cellular signal?

The application sometimes takes a bit of time to refresh the information it reports about devices. I run it on an Apple iPhone 12 Pro and I find the mobile application to be a bit finicky myself. I have the Nokia gateway so I don't often bother with the mobile application. When I do it is always an argument.

With a laptop connected to the network you can open a terminal/console interface and get an idea about the other devices it is aware of. If you have the console open and issue the command ping 192.168.12.255 that sends ICMP packets to the broadcast address of the LAN and all clients will see those. Your client should get responses and then you can issue the command, "arp -a" which will list the ARP entries in the clients ARP table. There will be a listing of different IP addresses and each client's hardware address. Each IP address is of course a different client recorded in the table. If they are alive and active they should respond to the pings and then the client that issued  the ping to the broadcast address should provide the listing. If the count of devices is as expected to the number you have well there you go. You know they are there and respond.

Are you saying the devices are all offline as they do not connect to the wireless LAN or that the mobile application does not report them as online? The mobile application may not report all the devices right away. That is how the mobile application seems to behave for me here. It will eventually report devices but not right away. That is assuming I can get the mobile application to work and let me have any options at all. 

I am writing this on my MacBook and the application on the phone finally let me login but it reports no cellular connection and NO devices so yes I get that. It is now doing the spinning doughnut and is not responsive. So, that I often see with the mobile application. Not a stellar application especially on IOS.

iTinkeralot wrote:

Which gateway do you have? The Sagemcon I am guessing. Does the LED display on the front of the gateway have the bars displayed or is there NO cellular signal?

The application sometimes takes a bit of time to refresh the information it reports about devices. I run it on an Apple iPhone 12 Pro and I find the mobile application to be a bit finicky myself. I have the Nokia gateway so I don't often bother with the mobile application. When I do it is always an argument.

Nokia 5G21