Tmobile 5G Internet Gateway doesn't connect with anything old? Terrible customer service?

Depending upon the age of your iPad its authentication type (WPA, WPA2, WPA3) is probably either WPA or WPA2. The other factor is the capability of the wireless module inside the iPad. It really depends upon the generation of the iPad. Currently there are 9 generations of "iPads". You can look at the very fine print on the back of the iPad toward the lower part of the back of the iPad and see the info more than likely with magnification. https://support.apple.com/es-us/HT201471

That can tell you the model and then you can identify how old it is. If it is a 5th generation or earlier the wireless connectivity may make getting it connected a bit more of a challenge. My guess is you probably need to get someone (from T-Mobile support) to assist you to create an additional SSID, i.e. 2.4GHz network ID on the gateway and establish/allow the WPA/WPA2 authentication types to get it to connect. Your iPad can only do what it is capable of so the gateway ALSO has to present that option to the iPad to get it connected. 

My guess is that the iPad is old enough that it only supports up to 802.11n wireless and the only WPA or WPA/WPA2 authentication. It is not uncommon for older wireless devices with earlier versions of wireless capability to be a challenge to get connected with the T-Mobile gateways that have the newest released version of wireless technology. 

The terms WPA WPA2 WPA3 refer to the encryption type. If the iPad only speaks/translates WPA encryption type then the gateway deben present that option to the iPad. If the gateway only presents WPA2-WPA3 and the iPad can only use WPA encryption it cannot play. The other aspect is the version of the wireless capability of the iPad which, if older, will need a bit of extra attention. 

Another option is to take a wireless extender and connect that to the gateway and then present the network name (SSID) to the iPad with authentication it can translate with. I am sure if you can find someone that has some technical knowledge of wireless to help you you will have a good shot at getting it connected. If you have the model number I can look that information up and determine the age and the wireless capability of the wireless module in the iPad. 

Depending upon the T-Mobile gateway you have that also will dictate the process and steps required to make the changes to the gateway to accommodate the iPad for connectivity. Is the gateway a round cylinder with a small LED on top of it or a small black tower with rounded corners and an LED display on the front. That is also necessary to know to provide more details as to how to get it done. If you reach out to T-Mobile support and can get an agent that is really helpful you have a shot at getting it done. 

I am happy to try to answer questions and I will try to keep them clear so as to not make the tech too daunting. With networking and todays technology it is hard to not speak in tech terms.