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

I run my Asus off the Askey just like I did my Netgear modem when I was on Spectrum... auto Config options for the WAN and all picked things up just fine.

Even IPv6 Passthrough seems to work... somewhat.  At least my phone uses it fairly consistently.  Windoze stuff can be kinda squirrely when it comes to IPv6.

Our market got 5g up after the shortage hit for the Nokia modems, so we are still on the Askey LTE boxes.  Even while connecting things directly to their modem by wire or wifi, DMZ and port forwarding (via UPnP or manual rules) would NOT get around that double-NAT behavior.

That box's wifi is pretty lackluster though, so I disabled it and use my Asus instead.  Other than that, my network "worked" out the box just like it did on my Netgear did with Spectrum as far as getting the uplink to the internet.  Only caveat is the whole XLAT/CGNAT mess that breaks P2P and the like.

No changes were needed for my local network, since everything was already looking to my Asus as their gateway for DHCP and such.  I literally could just run the Asus to the TMO LAN 1 and reboot it to resolve things via DHCP.  I just went the extra bit to change the TMO subnet to match my Spectrum numbers and password so my existing shortcuts would work. We can actually turn off their firewall and all... basically making it behave as a dumb passthrough (just not an actual bridge mode).  But it makes no difference because of the screwy crud they are doing on the upper layers.

No matter how configured the Askey while wired directly to the TMO LAN 2, I couldn't get a fully open NAT for the PS4.  Best was NAT2, and things were just still broken because of TMO's topology.  Even using DMZ made no difference.  So I put it back on my Asus and just run their modem as a dumb device, basically.  So everything is still managed by my Asus, just as it was with Spectrum.

No matter what you do, things will still actually behave as a double-NAT scenario because of that xlat/CGN crud they are doing.

Dug around and figured out I could likely fix that with Windscribe VPN running on my Asus.  But even on annual plans, setting up their cheaper data center option to get a more static IP and up to 10 ports forwarded, it would be about the same cost after haggling with Spectrum for a discount each year.

If I get to the point I NEED that full functionality again, I will likely just flip back to Spectrum at that point IF TMO hasn't fixed their networks.  The extra cost won't bother me as much since it will also give me the benefit of better routing to reduce pings.

But for now... streaming and playing casually on the PS4... I am sticking with the cheaper TMO option to give Spectrum the 1-fingered salute for a while.