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

TMO HI is basically CGNAT, with a sprinkling of psuedo VPN mechanics in play for their XLAT464 implementation to work... so you can't really get around the double-NAT scenario unless some MAJOR changes occur at higher network layers.

Also keep in mind if you are not linking the routers over ethernet, you will get that weird half-duplex wifi management that cuts throughput in a major way.  Also want to make sure that ethernet link is not registering as 100mbps.  Some routers have LAN or Switch management options that can impact realized throughput as well (may be an acceleration toggle, or something mentioning Cut Through Forwarding (CTF), etc.).  Sometimes this needs tweaking to actually achieve gigabit throughput all the way through the router.

About that TP link network... are all the devices using DHCP, or are there a lot of staticly set addresses in play?

You should have the option of running your router in access point mode.  This would allow everything to coexist in the same subnet, and if DHCP is in play they should behave fine, provided you bind their IP's in the gateway managing DHCP.  There are drawbacks of course, like nixing routing features in the TPLink, but it is an option that would still allow your TPLink wifi connections to work if it provides better/more consistent coverage than the TMO wifi.