inverse portmapping?

Is it possible to define, in the web GUI config (of a RBR850 (AX6000)), a port-mapping that goes the other direction than normal... ie. from LAN to WAN rather than the normal WAN to LAN? Or perhaps I should be using a sort of NAT Loopback ?

Here's why I'm interested in that:

My Orbi - a RBR850 (AX6000) - serves a LAN of 10.0.0.x/24 just fine.
Those LAN clients can talk to each other, access the Orbi web gui, and (naturally) reach out into the internet.
The Orbi itself is connected-wired to a Mobile GSM(SIM) router - a RUT240 - and *that* (mobile) router gets internet from the mobile cell network.

So the Orbi is the only "client" on the RUT240 LAN (192.168.1.x/24)
All other clients are on the Orbi LAN (a subnet 10.0.0.x/24),.. routing through the Orbi (10.0.0.1) (the Orbi as 192.168.1.2 then routes through the RUT240 192.168.1.1) out to my mobile phone service provider.

I would like to connect, from inside the Orbi LAN, to the RUT240 web interface.
When I try to do so from the Orbi LAN, the RUT refuses the connection because the client is not on the local LAN (the local lan to the Rut).
Note that clients (on the Orbi LAN 10.0.0.x/24 routing through 10.0.0.1) are able to successfully ping the upstream router RUT240 LAN IP (192.168.1.1) however the Web GUI on that RUT240 only accepts connections from local LAN clients (directly) and not subnet clients on the Orbi LAN) <sigh>.

Is there a way to configure the Orbi,..so that a local client (on the Orbi LAN) appears as coming from the RUT LAN to which the Orbi is attached?

Conceptually, I'd be ok if I could Port-Forward, on the ORBI from the internal LAN out to the external WAN.. so that I can reach the RUT.
Anyone got any ideas?

Thanks for any thoughts.


p.s. Perhaps I might tell a local clients (one the 10.0.0.x/24 LAN) to route through 192.168.1.1 (the  mobile router as seen from the LAN side of the network of the RUT). Hmmm.

p.s. I've heard of something called a  "NAT Loopback". Is that that I should be looking into? I read, "access of a service via the WAN IP address from within your local network.". Indeed, the WAN side of the ORBI is the LAN side of the RUT240.