Router rebooting

You don't refer to which gateway/router you have. T-Mobile has the Nokia, Arcadyan, & Sagemcon gateways. I have seen a couple of reports of behavior like you refer to recently. It might be related to a bad power adapter, software issue, or a internal electronic problem on the board. It is hard to say for sure. 

Not knowing about the steps taken when you obtain the gateway and set it up it is hard to say for sure. While watching one of Nater Tater's videos he made a very good point about the initial setup of the gateway. Since T-Mobile pushes out the firmware updates as they do it might be related. He suggested to plug in the gateway and allow it to stand idle for a bit of time prior to doing the initial configuration and setup of the local wireless network so in the event a firmware push was made and loaded it would NOT be interrupted in the middle of the progress by accident. With the Nokia gateway the small LED screen is on the top of the gateway so it is hard to keep up with what is taking place on that one. With the Arcadyan and Sagemcon they have the LED display on the front of the gateway but I do not have one of those so when a firmware push is made I do not know if it posts a warning or notification that the device is in the process of updating and to not interrupt the progress of the upgrade. 

Given there may not be an official T-Mobile statement about this possible situation upon the initial setup process I suppose it is possible to stumble into the problem. If the gateway does take a firmware push and is in the middle of an update and it is interrupted at a critical point by a power cycle there could be corruption and result in the gateway being able to recover and boot to a functional state. It may be there are proper checks in place for recovery but maybe not. Just speculating as to how it might get into such a state IF there is/was no hardware failure. 

So, there are probably a few possible ways the gateway could get into that state but without being able to debug the device it is not going to be clear. I think your best bet is to take it to a local T-Mobile store and request a replacement. Then when you get a replacement home just power it up and allow it to stand for a period of time or say 2-3 hours. It would probably be safe to check the initial state for the firmware revision and record that and then after a bit of time check it again. If the firmware did update then leaving it set so it can load the newer firmware is probably a good thing. When Nater Tater was doing his video with the Sagemcon he was reporting on it was a brief time before it did report the firmware update was started. He waited for that to complete before proceeding and that was when he made the reference to watch for that and not to interrupt the process. 

Just a theory but it is possible it might be related. 

I have this problem now, I'm supposed to be getting a 3rd Sagemcom Saturday.  Had the arcadian first, worked fine until mid november and it rebooted randomly.  Tech had me try a factory reset and it never came out of a boot loop.  Next sagemcom lasted just a few days before it started rebooting itself on a regular basis.  This last one has been intermittent.  Factory reset seems to make it better for a while and then it happens again.  So frustrating.

I had the same rebooting problem with a Sagecom gateway after about 3 months of use. I exchanged it for the ARC brand about a week ago. So far, so good. We'll see what happens. The Sagecom gateway also suffers from a design flaw - the USB-C power connection is poor and the USB-C plug doesn't securely fit into the receptacle. I had this issue on two Sagecoms. 

I have the Sagemcom Fast 5688W It ran the firmware update fine.
 

The first one I got mid September ran fine from then after a few weeks began rebooting 20 times a day and eventually it got to a point where it would just be stuck in a reboot loop. 

This current one (my second) was fine until this past week and it began to reboot once or twice a day then began rebooting again once every few hours. Today it has rebooted once every 20 minutes. It's beginning to act like the first one. There seems to be no explanation or way to correct the issue. 

The power cable is fine and hasn't been moved or touched and is plugged into a UPS power supply unit.

Reset the router? SIM card bad? 

Try reseating the SIM card. Of course power it down first. I had heard a few reports of issues with the Arcadyan gateway and the SIM holder. It is worth a shot. 
I have not used the Sagemcom firsthand so can only follow the various conversations. It might be bad hardware or a software issue. Hard to say. 

I just received my 2nd Sagemcom device today after experiencing the same power cycle/reboot issues constantly. Curious to see if other experience similar or if 2nd device suffers same fate after a while. 

It is so disappointing to see the repeated failures and attempts to get resolution on these issues. Sometimes it seems the replacements are succeeding and then not. T-Mobile needs a much better solution. Difficult choices. The current model for dealing with this doesn't seem sustainable to me. 

I have this issue as well and am considering purchasing an AVR (Line Interactive) battery backup incase the modem is just really sensitive to voltage spikes and drops in the house.

Does anyone else experiencing this issue happen to have a UPS (Specifically a Line interactive (not a standby)) one their gateway is behind?) Wasn't planning on purchasing the UPS until next month but can reprioritize it if needed.

I cannot believe I am saying this, but my issues seems to have (mostly) gone away over the holidays.  I am not sure if that is due to less traffic with more people in the area traveling potentially, all the leaves are off tress to clearer direct line to tower, etc.  The final theory I have (and I am begging someone to verify and explain) is that my wife put a small, metal, decorative Christmas tree (think 10 inches tall or so) on the mantle directly behind the device.  I put the decorations away New Year's Day… and the internet dropped.  So put the tree back and I am content to leave it there.  I have heard elsewhere (reddit forums, google searches. etc) that people have used foil to somehow improve device performance.  If for me, that is a Christmas decoration that stays up year round, so be it.  I am not now willing to remove for either true performance or plain superstition.  

How locked down this device is has NOT helped with my troubleshooting, 

Has anyone tested or had experiences with the following possibilities?

• Could the Gateway be unstable due to power/voltage fluctuations coming from the outlet? (power will normally fluctuate throughout a day, especially with older wiring or when major appliances kick on) Some electronics, especially older modems and routers are VERY sensitive to this.
• As the Wifi Channel (not to be confused with band) cannot be set, could the gateway be periodically changing the channel to one it thinks is less congested, triggering a reboot?
• Could Celluar signal fluctuations be causing a drop?