How do I change the IP to other IP than 192.168.12.x?

Its not a big deal … your internal ethernet can have a completely different network numbering scheme, mine is 10.0.0.x than the modem’s unchangeable silliness. And whom ever said it, you are completely correct, support hasn’t the vaguest idea what you’re talking I have both my own self built pfSense, and Untangle, now Arista (the free version works perfectly and is easier to work in than the pfSense one). You just set up and internal and external network, obviously you need to NICs, and tell the “external” one it's a WAN, and set it to DHCP, then set the other NIC for your internal (LAN) network. Arista cleverly figures this out, an you don’t even have to put any routes in. Boom, simple, not need to fiddle with useless “engineers” at T-Mobile.