Newbie to TMHI...so what are we supposed to do with IPV4 devices?

The T-Mobile gateways DO provide both IPv4 and IPv6 addresses for clients. The IPv4 is translated across the IPv6 network and back to IPv4 but that is outside the gateway and the local network. The local network is 192.168.12.0/24 and has 254 addresses. The DHCP scope "appears" to run from address 101-254. I have monitored the IP addresses of the clients on my LAN and all have addressing above 100 so it appears the DHCP scope is only in the upper part. I have hard coded below 100 and had no issues as I have not stepped on another address. Of course the 192.168.12.1 is excluded from the scope and it is the reserved gateway address. There is also IPv6, assuming your client has IPv6 enabled and you would get the IPv6 link local address plus a few more. By default most Microsoft and Apple solutions have dual stack enabled. If you add the router and need IPv4 and IPv6 to the clients then you need to consider that as well. If you leave the T-Mobile gateway functional and add the router and add clients to its wireless vs the T-Mobile wireless and it works great. If you find it does not work then all you have to do is move the clients to the T-Mobile wireless or wired connection and remove the additional wireless router. Before you get too committed to the additional router and strip back the T-Mobile gateway it is easy to experiment and back up if need be rather than starting over with everything.

If you have a router you can connect the WAN port to one of the Ethernet ports on the gateway but do be aware that if you leave it as a router it will be doing NAT so you end up with a double NAT but it will work for most things. Putting the wireless router you have in bridge mode is another option. Some users disable the wireless on the T-Mobile gateway when possible and just use their own wireless router or mesh solution connected to the T-Mobile gateway. I am pretty sure if users do an online search there are resources that explain it and probably YouTube videos where a geek is just having fun sharing the experience. The information is out there. Adding an additional router is just a bit more effort. 

Using a router that you have which you know the IOT devices connected to easily is a reasonable workaround for getting those connected. Some older devices like 6-8 year old printers that no longer have driver support do not play well with the newer WIFI-6 solution on the T-Mobile gateways. If you have an older client with an old wireless adapter it is much easier to buy a 802.11ac or 802.11ax USB adapter and disable the old adapter that does not play well or is slow. The 802.11ac USB adapters are pretty inexpensive and much better than fighting with an old wireless adapter which also has weaker authentication protocols.