fast 5688W gateway is cutting in and out

Same/similar. Box reads:

SAG FAST5688W 5G GATEWAY BLK 

Did it's own firmware update on first power on. App shows: 

Información del Gateway

Versión de firmware

SGJi10105-1.1.52

Shortly after getting established, the following Friday (last week) it dropped everything (including Wi-Fi) 4x inside an hour. Wi-Fi seems ok since then, but it has dropped Internet WITHOUT REPORTING SAME both 3 days ago and 4X AGAIN this afternoon. The biggest issue I gave with that (at the moment) is the complete and total failure of the gateway to recognize and report the issue. Connected Wi-Fi devices (e.g. Xbox One S and X) will not report a network loss, but also not show any active network activity (active downloads in progress will simply halt).
 

Not being a network engineer, but having significant networking knowledge and experience, it appears that the gateway is using a method other than successful network traffic to judge the state of the connection, which is completely inaccurate and a major design flaw. As a result, the gateway fails to recognize the lack of a properly working connection and fails to  follow any kind of automatic retry procedure to re-establish a stable, working connection. A power cycle reboot of the gateway will re-establish connectivity after the usual excessively long delay, but within an hour it's back to "showing" connected but not actually being connected.

This is basic low-level networking 101, and the gateway is failing miserably. This needs to be investigated and resolved ASAP. These continuing issues are only going to tank T-Mobile's reputation.

I've complained about the same issue, and they give a lame answer about heavy load on the tower. I think I'll start logging the reboots and ask for credit on my bill for failure to provide service.

The TTL setting on the network traffic is too short.

If the gateway reports the cellular connection is up then the problem podría estar with the DNS. By default the gateway will install the gateway address 192.168.12.1 as the DNS source. You can go to any client you have on the LAN and set it's DNS to 1.1.1.1 or 8.8.8.8 or 9.9.9.9 and then see if it starts working. If it starts getting communication to the internet sources then the problem is with the DNS. T-Mobile's FAQs clearly state they allow the change to user's DNS. The gateway DNS is not alterable but clients are.

If you go and shorten the TTL value it can influence the caching of the DNS information but some DNS servers will not accept or will reject a low setting. if the TTL is set to 0 it is not defined in the standard therefore it will probably cause problems with the DNS. Tuning or changing TCP/IP values without knowing how it is supposed to work will indeed break things or make them worse. Changing the client's DNS server(s) will not hurt anything and can often work around a failure of the current DNS resolver. 

It is easy and quick to change the DNS source value so that is one quick datapoint to check and rule out. Since the NAT interface external IP address changes it is possible T-Mobile uses DynamicDNS and there is some setting that needs to be addressed. It could be the gateway is just not telling the DNS server it needs new information when its external IP address changes. Just theory as I am NOT a DNS expert. 

Is anyone else having issues with their works vpn connection. I work from home often and with the 1.1.52 update I have been having issues connecting my VPN to the it? Internet speeds are great but, no connection.

Actually, I called again and got a person who really listened to my description of the problem.  She decided I had received a faulty gateway. She sent a new one, and it's been working well ever since.