There is no access to both gateways, the almost totality of LTE providers don’t give you a public IP, they do the first level of nat. Starlink does also the same, in all these cases the IP that you see in sites like whatismyip.com is not a public IP and you cannot update a dyndns service with it.
My old slow DSL that I used for years has been discontinued by the provider because of bad old phone lines that they deemed too expansive to replace. They proposed a FWA that is also natted as Starlink and LTE modems.
I am sure I am not the only one with this problem, do you have any plan to provide some cloud access, even if for a fee? It’s a huge limitation otherwise, only option is to remotely access a LAN PC/raspberry via VNC/TeamViewer/Anydesk/etc and log from there.