There is an Internet from the modem and from another router. How to set a specific interface for accessing the Internet?

Question

linux

There is an openwrt router (tpl-mr3020 v3).
Two Internet sources are connected to it - a cable is thrown from the main router and inserted into the rj interface. And the USB modem is inserted.

There is a need to access the Internet, he could only through a modem, and the connection that was used by wire was used only to be able to connect to the router itself (I raised a proxy on it, and for the sake of convenience).

At the moment, I plugged everything into it, set dhcp to receive ip in the rj interface, set a proxy, inserted a modem. I tried to set various traffic rules, but there is not enough knowledge.

At the moment, when connecting via wi-fi to it or when connecting through a proxy, the Internet is used, which is distributed by the router, and the modem seems to be ignored.

Answer 1

find the IPv4 routes section
there, find the lines that have Target 0.0.0.0/0 (there should be two such lines - for the modem and for the cable);
Metric for the cable interface, set more than Metric for the modem.