How to reach from one subnet to another on the same device?

Question

Computer networks

Hello, I have a Keenetic router, two Internets are connected to it, one via a cable, the other via a 4g modem ErgoW02
Network on kinetics 192.168.1.0
And the modem has its own network 192.168.3.0
When I work via cable everything is super, there are 3 devices on the network, I turn to them from the internet just by forwarding the ports accordingly for each
But how do I access these devices when the internet via cable is disabled. because the devices are in subnet 1.0 and the modem creates its own network on top.

In the settings of the kinetics, I did port forwarding for both connections, but it does not help.

Answer 1

Your modem is not visible from the Internet, because does not have a "white" IPv4 address. Solutions (offhand):

1. Pay extra to the mobile provider for a "white IP" (if there is such an option);
   
2. Use IPv6 (if supported by the provider and firmware);
   
3. Establish a tunnel to an external node that is visible from the Internet and forward ports from it.