You can find the IP of your NAS by using the T-mobile Internet Gateway app and look under the devices app.
To add to tomwil’s answer - once you connect your NAS you can you the t-mobile internet app (not the regular account app) and see your NAS and its IP under the devices tab. I would be nice if you could make this a static IP but doesn’t seem to be possible
Already have an account? Login
Enter your username or e-mail address. We'll send you an e-mail with instructions to reset your password.
Sorry, we're still checking this file's contents to make sure it's safe to download. Please try again in a few minutes.
Sorry, our virus scanner detected that this file isn't safe to download.