For Ethernet, yes the port acts as a LAN (access using the 3G connection) or WAN (used to provide Internet to the device). So this would work fine.
For the public IP, this is the tricky part and conjunction with the port forwarding. There are many devices like the one I linked and just used that for illustration purposes. However I would double check the product manual/website to confirm it has the ability to port forward, etc. Verizon is the only carrier I know that sells public IP’s which are a one time fee of $500, I believe. I really don’t know anything about T-Mobile though.