I have a problem similiar to this
I have connected a device to my LAN port which requires static ip address
I have given the LAN port a static IP address by modifying the dhcpcd.conf file and adding
to it. Now, I can connect the device on LAN port if I type the 192.168.1.2 IP address in the browser. But when I use a 3G USB (eth1) dongle to connect the device (Rpi) to internet, I cannot access the LAN port. The USB modem and LAN port work fine independently but not simultaneously. It seems like the device on LAN port disables the USB 3G modem.
I found this question (Raspberry Pi Connect to 3G modem instead of ethernet) a bit related to my post, but I couldn't understand it properly. My knowledge of LINUX is very limited. Can somebody please help? Fyi, I am running Raspbian on RaspberryPi 3B.