Phone not automatically connecting to 5g internet

Um. So, I've had the T-Mobile 5G Gateway for just over two weeks now. And I've discovered stuff.

First off: Regarding phones. Say that, before one got the 5G Gateway, one was running a router or something with a particular SSID. And said router would have been a decent router and all that, whether or not it comes with the cable connection is or something that one had bought before.

Now, with said pre-existing router, one would probably have been running with IPv4 (typically: 192.168.1.xxx addresses), with WPA/WPA2, or, possibly, WPA3. And it all would have been working.

So, first things first: Remove the cable modem, put in our handy 5G Gateway, fire it up, and, lo and behold, it kinda works. Now, one thing the app does right off is suggest that one go ahead and set the SSID and wi-fi password. No surprise: When I got the instructions to do that, I hauled out the SSID I was using for the dozen or so widgets in the house that had pre-programmed in (printers, the Chrome TV dongle, the Roomba, the PCs, and the phones) and used that; and, naturally, used the same password as was being used for that pre-existing SSID.

I had, shall we say, interesting problems. And now I cut to the chase:

• The 56G Gateway is very definitely positively using IPV6. It does IPV4, too, but it's kind of a sideline.
• The 5G Gateway very definitely implements WPA3, as well as WPA and WPA2.

The dumb Internet of Things (printers, Roombas, and the like) don't have a clue, or much of one, about IPV6 and certainly don't know about WPA3. The cell phones and PCs, which are being kept up to date by various and sundry updates, Got Confused. And either wouldn't connect, wouldn't stay connected, or, if connected and got turned off/turned back on again, wouldn't connect without having to put the wi-fi password back in again.

There turned out to be an easy solution. On both PCs and cell phones, it is possible to see the networks that one has connected to at one time and that the device has memorized. On an iPhone (for example), it's Settings->Wi-Fi->Edit (top right corner), at which one is given a list of every wi-fi network one has connected to. On a Windows 11 PC, it's Settings->Network & Internet->Wi-Fi->Manage known networks, which comes up with a similar list.

DELETE THE SSID FOR THE GATEWAY in the PC/cell phone/tablet.

Then, go back and connect the PC/Cell/Tablet to the SSID, providing the password when prompted. Ta-Da! No more connection problems.