Unable to access RTSP stream despite my mobile device being able to access the feed?

It's possible that your IP camera is having trouble with your default network. Here's something you can try that has worked for a lot of people with security cams, smart switches, printers, etc.

The first thing to check is to make sure the Hidden switch on your network is set to Off. By default, the Hidden switch is On for the primary network. Use the T-Mobile app to access the settings for your network. Tap the Network tab on the bottom, then tap your existing network. Scroll down to the bottom of the page and you'll see a Hidden switch. Turn it off and then Save.

Now, try adding an additional, new WiFi network for your IP cam's use only. Use the T-Mobile app and tap the Network tab on the bottom, then the "+" button on the lower right. Create a new 2.4 GHz WiFi network with a unique name (SSID), just for your cam. Make sure the WPA type is WPA/WPA2. This is different from your main network which, by default, is WPA2/WPA3. See if that doesn't allow your cam to connect and behave the way you'd expect.

For the record, very few of the devices I mentioned above are capable of using either WPA3 security or the 5 GHz network. By creating a second network that only broadcasts on the 2.4 GHz band and has the lowest security with WPA/WPA2, devices that can't support the latest protocols can still use your WiFi.

¡Buena suerte!