Port forwarding to different subnet

[bradmca2021]

I have a static public IP - Vodafone router (THG3000), ASUS AiMesh routers at home. 

The Vodafone router is the 192.168.1.1 gateway, Wi-Fi off, just connecting to the main AiMesh node which has DHCP WiFi etc running and is on a 192.168.40.x network along with all my devices.

The above works very nicely.

What I would like help with from some clever networking people is how can I port forward to make one device accessible from the internet. It seems the 192.168.1.1 router isn't allowing mapping to anywhere other than 192.168.1.x - am I missing something? - could I allocate the AiMesh network to a LAN range of 192.168.1.2-255 ?

Thanks for any suggestions you can offer

[CrimsonLiar]

Yup, doing that with an OR ECI modem is just a touch more complex, and probably only something to do if you have a spare modem.  As it's the Lantiq VX268 (from memory) it's the superior chipset but only so long as it's working...Right, so where's my programmer...