2.4 Ghz internet devices keep disconnecting from GSE internet box

I like all these steps.  Unfortunately it is nothing new that we already tried on my first call to tmobile support.  Which ended in getting a new modem shipped out even though the one I had was only a week old. The issues started after a forced firmware update or that is my suspicion. 

So far multiple factory reset.  Sim exchange. Always breaking up the network into multiple separate bands and dumb down the security protocol.  Daily restart of the new replacement modem because the tmobile internet app itself won't even work unless you frequently restart it. All 4 bands sit up as you described with one that is combo of 5 and 2.4 ghz (default from the box), one only 5 ghz, one 2.4 ghz dedicated with the WPA 2 protocol and one 2.4 ghz only with the WPA 3 security.  Unfortunately 4 is the max these router will support.  I have trimmed back to 3 and even 2 bands only in case I was overwhelming the router but that didn't help either.  Even as I wrote this help request up last night I went down the hallway and noticed my thermostat was no longer online (yellow exclamation mark across the wifi symbol, usually means signal is reaching it but the password couldn't be negotiated). 

My home isn't big so from the router to the furthest device (smart TV) the signal strength is 80 percent on 5 ghz and 100 percent on 2.4 ghz (because like you said, 2.5 travels further than 5 ghz) and is about 30 feet away from the router.  The router itself is speed ping test at around 216 to 250 Meg down load and between 116 to 140 up so it's able to handle multiple streams without lag. (I have recently added an external antenna to get those speeds and was hoping that would make a difference in problem even though it shouldn't and it didn't). Problem still exists regardless of internal built-in antenna or the external antenna. 

My biggest clue so far is how the printer was assigned the wrong default gateway or other times the wrong subnet mask when connected via Wi-Fi. Other devices are not able to provide that information to me but the printer has a  trouble shooting page that prints the network status in all the details.  Since this is assigned by the router DCHP function it seems the issue originates there.  Here's another anomaly that happened on both routers so far.  The LG fridge shows as being connected to the network but without the Wi-Fi symbol which usually means the device is connected via ethernet cable. (Amazingly sometimes it does have the wifi symbol but regardless of the presence or absence of the symbol I still only manage to have a  connected device for just a few hours before it looses internet functionality) 

A new puppy is what caused all the problems as she dug up the cable connecting my house to the Comcast line pole so I figured I would give a wireless router a try. Since I am in a wheelchair it would be hard to dig a new trench in the yard and encase the wire inside piping that she couldn't chew through. I even would have to go under a small area that has been covered by concrete so that's why I left Comcast. It is not impossible though I would like that as a last resort answer. 

Please let me know of any other ideas I didn't mention as trying already. And thanks for your thorough explanation of the steps suggested 

Norman