When can I JUST use TM internet modem as ONLY a modem, in BRIDGE mode, with NO NAT, NO firewall, and frankly NO Wifi.

Right on!  I'm on the same boat as you are.  I bought a Cellular Modem/Router and got an Internet-only plan with TM.  The set up works fine if you are only using it to access the Internet from within.  I found out that, after setting up DDNS, L2TP VPN Server, Port Forwarding, etc., I cannot even access my LAN from remote!  AND there are two different IP addresses!  There is the so called "Public" IP Address, then there is another "IP Address", and when I do whatismyipaddress.com, it gives me another IP Address!  The DDNS complains that it cannot do the job because - here is the exact wording: "The current external IP is not a public IP, and the DDNS service may not work properly.". This is nuts! Why are there so many IP's. I cannot access any of the three IP addresses from outside.