Issue with creating 2.4 network- plz read entirely

Nc1037 has the right idea. However, I kept getting an error message. So I created a new network with a “Frequency band” of “Automatic (recommended)”. It created a second 5 GHz network, but after it was saved, I went back and changed it to 2.4 GHz, saved it again, and it worked! Other settings are “WPA2/WPA3 (recommended)” and “AES (recommended”. I was able to connect my cameras and smart plugs to the 2.4 network with no problems. Good luck!

CORRECTED for newer version of the T Life app (I don't see an option to edit my previous post.)

T Life app instructions to create new 2.4 GHz network:

Tap Internet (at the top). Tap My Wi-Fi (at the top).

At the bottom, tap + Add new network.

General details section:
Enter a Wi-Fi network name (SSID) (TIP: this should be alphabetically after your primary network name.)
Enter a Wi-Fi network password.

Advanced settings section:
Frequency Band: select 2.4 GHz
WPA Version: select WPA/WPA2
WPA Encryption mode will be automatically set to TKIP+AES.

Tap Save at the bottom. 

I recommend using the original default network configuration as much as possible. (Automatic frequency band & WPA2/WPA3 security.) Use the second 2.4 GHz network for devices that have trouble with the default network.

T Life app instructions to create new 2.4 GHz network:

Connect (at the bottom) > Things (at the top) > tap your Gateway > tap Manage > under My Networks, you will see each one you already have. 

To create a new network, tap Network (at the top.)  Under My Networks, tap + Add Network to open the page to set up your new network.  Give it name (SSID) and a password; select the frequency band (2.4 GHz); select the WPA version (WPA/WPA2).  (Encryption mode is automatically set when you select a WPA version).  Tap Save at the bottom.

Hi Caroline,  Seems I have the same problem with a TMO-G4AR 5G router/modem.  None of my Windows 10 laptops or tablet are able to receive the 5G wireless signal.  I'm forced to access the internet with ethernet cable only to my devices.

T Mobile support told me that this modem can be altered (configured?) to send a Wi Fi 2.4 G signal.    The online user guide mentions 2.4 as well but without any details.  I've been to three T Mobile stores to ask in person, but have yet to find a staff who understands the question.  

Since both inquiries of mine came to a dead end, I'll have to start the phone support again because now I've come across the possibility of connecting a 2.4 router to the TMO-G4AR by ethernet cable to create a 2.4 signal.  I'd be interested in any progress you might have made on this front.  Usually, to access the internal settings of a router/modem you enter an I P address into a connected computer's browser.  Not in this case, it seems.  Access to the settings of the T Mobile modem it through an app provided by T Mobile installed on my T Mobile phone.  I wonder if you've tried it.  I did, but found it frustrating.