My new 5G gateway shows excellent connection but internet speeds are slow.

I am not an engineer, but often the problem is with the tower you're connecting to for service. The gateway may show that you're connected with 4-5 bars, but there is either congestion or interference on that tower affecting your speed.

How you connect, e.g., the number of bars, isn't always tied back to download and upload speeds. It's like WiFi if the Internet goes out. You'll still see WiFi on your devices, but there's no Internet behind it because the source is down.

You should talk to T-Mobile support and see if a problem or maintenance is causing the difference in performance between the two locations.

¡Buena suerte!