IP address

NO. The private Class “C” network 192.168.12.0/24 is established as the common host IP network on the gateways and users cannot change it. The DHCP server provides addresses, from what I can tell, from 101-254. The gateway address is excluded from the host pool range and the addresses 2-100 do NOT appear to be included in the DHCP pool of available addresses. T-Mobile only supports 64 clients on the network. Some of the gateways have defaulted to 128 but actually the official number is 64 according to the documentation I have seen. 

If users connect another router with its WAN interface to the gateway Ethernet it is possible to run your own private IP subnet behind the NAT of your router. Just be aware double NAT can present some issues.