Buffering/weak signals in other rooms

You'll need to use the Orbi in Access Point Mode. From your description, it sounds like you just plugged it in and left it in it's original state that you were using with Spectrum.

The KVD21, as you have discovered, is not very configurable, and it will not allow you to customize or modify DHCP. If the Orbi has DHCP on as well, your network won't work well, if at all.

Also make sure you're using different SSIDs for your Orbi from what you've created for the KVD21. Using the same network names will also cause network problems. After making sure the network names are unique, you might consider making T-Mobile's network Hidden so that no one will inadvertently log on to it. You can do that under the Network tab of the phone app. 

Try changing your Orbi to Access Point Mode. That will allow the T-Mobile gateway to do what it needs to do and the Orbi will handle the WiFi distribution.

¡Buena suerte!