Use a different Internet provider or await a firmware update for the router and hope they add these features. Apparently T-Mobile have indicated they will be adding a bridge (pass through) mode to the capabilities which would allow you to use your own additional router internally.
In the meantime, yes you can create a static IP in the 192.168.12.x space that will work for you for internal LAN access.
External WAN access will likely be a problem. I believe there is an add on for HA discussed on this forum that offers a VPN like feature via an external provider that might work for you with a dynamic HA IP address and a dynamic WAN address. It doesn’t need port forwarding presumably because the connection is instantiated from within your LAN.
Maybe this one ? Not recommending as I don’t use it but it maybe works for you.
Hass.io Add-on: SocketXP - Home Assistant Remote Access (No DDNS + No Port Forwarding Required)?