Pre-purchase questions about the T-Mobile home internet

I can't answer your first question. You’ll need to talk to T-Mobile about that, but here is some info on the other two.

T-Mobile gives you two weeks to decide whether their solution works for you or not. In my case I had Xfinity and cancelled them within three days of getting my T-Mobile gateway.

Your third question is a bit more complicated. Assuming they give you the Arcadyan KVD21 5G Gateway, there is very little you'll be able to do with the configuration. There is no web interface as there most likely is with your router. All setup for the T-Mobile gateway is done via an app for either iOS or Android.

Your static addresses are probably on a different IP schema than the KVD21. T-Mobile gateways use 192.168.12.x for addressing. If you choose to use your router and it is also your DHCP server, you'll need to turn that service off. There can only be one DHCP server on the network and you cannot disable DHCP on the KVD21.

The wireless coverage on the KVD21 is excellent. I had been using a TP-Link AX50 with service from Xfinity, and the coverage is just about identical. My point is that there is probably no reason to continue to use a router once you move over to T-Mobile. Having said that, you could put your router into Access Point (AP) mode and use it for wireless coverage. The problem is that you can't disable the wireless on the KVD21, so the two wireless networks--your router and the KVD21--would need unique names (SSIDs). You would continue to use your router for wireless, but DHCP and addressing would be handled by the KVD21.

There are two gig Ethernet ports on the back of the KVD21. Your router could use one of those ports if you choose to continue using it. In my case, I decided to try it without the router, and so far I've been just fine. Full disclosure, I do use a WiFi extender in my bedroom where coverage is spotty, but that problem is not unique to the T-Mobile gateway.

As for static addresses, you probably don't need to do it, but if you choose to with the new numbering scheme, here’s how I'd do it. Change the devices that currently use a static IP back to DHCP. Hook up the Arcadyan KVD21 5G Gateway and let it hand out addresses to all of your devices. 

After the device has an IP address from the gateway, go back to it, note the address, e.g., 192.168.12.x, change the device back to static addressing and use that address. The subnet mask is 255.255.255.0 and the router would be 192.168.12.1. Not as easy as using a router, but it will work. 

The other option is to use an IP address between .2 to .99 for the last quadrant. The Arcadyan KVD21 5G Gateway reserves this range. You could give your printer an address of 192.168.12.10 and it should work. Unfortunately, you cannot configure DHCP on the Arcadyan KVD21 5G Gateway. T-Mobile wanted to make it as easy as possible to set up and maintain, and that’s the decision they made.

Also, there is no way to NAT or use UPnP with the Arcadyan KVD21 5G Gateway. It sounds like you're not a gamer, so that shouldn't be a problem for you.

Good luck and let us know how you make out. I'm a fan of my service from T-Mobile, both from a performance standpoint as well as cost. 

I agree, but over-the-air (OTA) Internet is only in existence since the beginning of 5G deployment, and it's a different animal from terrestrial Internet solutions. Having said that, there is NO reason that things like DHCP and static addressing can't be configured by the user.