Internet connection droped

It is possibly an assumption that the phone and the gateway both connect to the same cell. The reality is that they may not. The other aspect is that mobile phones get priority over the stationary broadband home gateway units. Another thing to consider is that the phone will often or probably use an IPv6 address and the home gateway uses IPv4 and IPv6. I have seen times when clients report no internet via the gateway but yes my phone works and still reports 5G. From time to time the DNS resolution from the gateway address 192.168.12.1 does not work as you might expect. To confirm if the reason is due to say DNS failing just go to the client network adapter addressing and change the DNS to 9.9.9.9 or 8.8.8.8 and then try it again. If the problem is not that the cellular signal is down but that DNS is not resolving then this can often resolve that behavior. 

Sometimes the problem is that the two devices are connecting via different cells or that the handset roams to another cell and the move is not noticed whereas the gateway does not tend to roam like a phone so it drops as there really is no signal due to service maintenance by T-Mobile engineers.