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

Port forwarding with the T-Mobile 5G solution seems to be a bit sketchy. They don't seem to advertise what will be forwarded and what is blocked. It makes it hard to determine if the solution is viable for specific case uses. From all I have read about the behavior it does seem to come down to the 464XLAT solution which is used across their IPv6 network. The support or lack there of that is included for port forwarding seems to be rather specific. It could be the marketing profile for subscribers is narrowed down to empower them to deliver services with the lowest overhead and technical complexity. Given what they are doing is rather involved it would make sense. Will they expand services in the future? That is hard to say. Maybe if market pressure is sufficient to push for changes. 

It is not all a T-Mobile problem. It comes down in part to vendors selection for service delivery and their propensity to lean on the old tried and true technologies based upon IPv4 delivery. If more vendors would develop their services with additional support for IPv6 the migration of services for IOT based systems over IPv6 would probably be improved and simplified. Given the ports you know are needed T-Mobile support, at least escalation level, should be able to tell you for sure if they will or will not forward those ports. If they are not forwarding them a VPN solution might offer a workaround but that is at an additional cost for a personal VPN solution and a bit more to set things up.

I've been able to connect my Sense home energy monitor to both my internal router and the TMobile Internet Gateway "Router", but Sense gives an error message "Network Problem - There was a problem talking to the Sense service."  Sense recommends forwarding TCP 123, 443, 6514, 8482, and UDP 53.

Any luck getting your Sense home energy monitor to connect to the Sense servers through your T-Mobile home internet, whether through the TMHI Gateway or your own router?

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. 

I also have devices that require IPV4. What are the steps to connecting a router to the gateway? I have a netgear router.

¡Gracias!

I don't know of any disadvantages, but am not that knowledgeable about the matter.  I do know that connecting my eero 6+ to the T-Mobile tower made everything I had easily connect to the internet again, after cancelling Cox.  That included a Lenovo desktop, Fire HD 10, Sony TV, Canon printer, Panasonic HomeHawk cameras and Moto T-Mobile phone.