Problem connecting gateway to internet through cellular connection

I'm by no means an experienced person in this regard. ### Context, if you find it helpful We are developing an IoT solution for telemetry purposes. The device that retrieve the data to send it to the cloud is a custom Debian (9) gateway developed by the same company that provide us the software that help us achieve this kind of things (Moxa). My problem is that I can't connect it to the internet through the cellular (LTE), so this is the reason I can't install libraries in the device (like the Google Cloud SDK). ## Situation The device has the following interfaces: $ sudo ip a 1: lo: mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1 link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 inet 127.0.0.1/8 scope host lo valid_lft forever preferred_lft forever inet6 ::1/128 scope host valid_lft forever preferred_lft forever 2: eth0: mtu 1500 qdisc pfifo_fast state DOWN group default qlen 1000 link/ether 00:90:e8:7c:e0:2e brd ff:ff:ff:ff:ff:ff inet 192.168.3.127/24 brd 192.168.3.255 scope global eth0 valid_lft forever preferred_lft forever 3: eth1: mtu 1500 qdisc pfifo_fast state UP group default qlen 1000 link/ether 00:90:e8:7c:e0:2f brd ff:ff:ff:ff:ff:ff inet 192.168.4.127/24 brd 192.168.4.255 scope global eth1 valid_lft forever preferred_lft forever inet6 fe80::290:e8ff:fe7c:e02f/64 scope link valid_lft forever preferred_lft forever 21: usb0: mtu 1500 qdisc pfifo_fast state UP group default qlen 1000 link/ether c6:d4:e8:d9:64:3e brd ff:ff:ff:ff:ff:ff inet 10.127.34.103/32 scope global usb0 valid_lft forever preferred_lft forever inet6 fe80::c4d4:e8ff:fed9:643e/64 scope link valid_lft forever preferred_lft forever According to the docs, the eth0 is for WAN and eth1 for LAN purposes. This is the content of my /etc/network/interfaces file: $ cat /etc/network/interfaces # interfaces(5) file used by ifup(8) and ifdown(8) # Include files from /etc/network/interfaces.d: source-directory /etc/network/interfaces.d auto eth0 eth1 lo iface lo inet loopback iface eth0 inet static address 192.168.3.127 network 192.168.3.0 netmask 255.255.255.0 broadcast 192.168.3.255 iface eth1 inet static address 192.168.4.127 network 192.168.4.0 netmask 255.255.255.0 broadcast 192.168.4.255 And this is the content of the /etc/resolv.conf. Those values are the DNS of the carrier (Movistar Peru): $ cat /etc/resolv.conf nameserver 200.48.225.130 nameserver 200.48.225.146 By the way, I've already set up the correct APN so the SIM should work (I did it through the admin panel of the gateway). This are the results of ping: $ ping 8.8.8.8 connect: Network is unreachable $ ping google.com ping: google.com: Temporary failure in name resolution Of course there might be errors in this config. What am I doing wrong? Thanks in advance. ---- # Update As requested, this is what the ip r/ip route returned: $ ip r 192.168.3.0/24 dev eth0 proto kernel scope link src 192.168.3.127 linkdown 192.168.4.0/24 dev eth1 proto kernel scope link src 192.168.4.127 Also, this is what the manual states regarding the ethernet interfaces: > The ETH0 Ethernet interface is used for the WAN and the ETH1 > interface is used for the LAN. The default IP addresses are: > > eth0=192.168.3.127 > eth1=192.168.4.127 And: > Interfaces of your eligible device could include: > > - tun0: VPN Tunnel // not using this > > - wlan0: Wi-Fi // Our required device doesn't include this module > > - wwan0: Cellular // > - eth0: Ethernet (usually LAN1 when the default setting is WAN). A diagram that might help: