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

Not having Bridge Mode is a PITA for me.  I need control over the DHCP to set the subnet and fixed IP's.  The old ASK gateway provides this.

Early on the ASK's had a problem with IPv6 over WiFi.  After a half-year of T-Mobile Engineering saying there was nothing wrong with it, they finally admitted it and fixed it.  But for a while I tried a Nokia.  It seemed to be a fine for WiFi setups and nothing else.  But the cell signal dropped 2 bars and was often unusable.  After going through 4 of them (one was DOA), I found that it locked to the LOWER level signal (happened to the the 5G) and ignored the stronger LTE.  Engineering was totally clueless, like usual.

So I went back to the ASK.  Fortunately they had finally (it took over a year) fixed the IPv6 WiFi problem.  So I was happy.  But it's no longer supported.  They can't even send me a replacement battery for it now.

I haven't tried it, but wonder if this might get around the missing Bridge Mode:  Use your own router which also supports a VPN for you LAN.  This way the TM gateway would only see the router.  You would probably lose IPv6, however, since most VPNs still don't support it.

Note: The 3rd generation gateway is just as dumb as the 2nd gen Nokia.  A support person tried to get me to switch to it, saying that it did support DHCP but I downloaded the manual and it doesn’t.

Note: Mullvad VPN doesn support IPv6 and it runs WireGuard which shouldn’t be too had to set up on a good router.