Use proxy server on keenetic to share internet from 4G modem

[Vladimir K]

Hi all 

Need some help from community to configure sharing internet from 4g modem using proxy server on keenetic.
 

Device model: Giga (KN-1011),
KeeneticOS version: 4.3.6.2

Three internet channels are connected to it (connection priorities, default policy, are in this order):
1) LAN-1
2) LAN-2
3) 4G modem

Huawei E3372h-320 modem connected via USB

KeenDNS is connected and configured

External clients need to be able to connect to the router via a proxy and have access to internet via the 4G modem, while the connection priority for users within the network is in the order specified above.

I installed and configured the 3proxy proxy server on my router.

I can connect to the proxy from the outside via KeenDNS.
user.keenetic.link:40097

I configured port forwarding from 40097 to 45697
45697 is the port the proxy is running on.

The problem is that when connecting through this proxy, the external client receives an IP from the LAN-1 connection, but the "4G modem" connection should be used.

The default connection policy should be left as is: LAN-1 is the primary connection, LAN-2, and the 4G modem are the backup connections.

How can I properly configure routing so that external clients can access the internet via the 4G modem when connecting through the proxy?

Thanks in advance for any tips and ideas.