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I've already set up my RPi 4 as a hotspot using dnsmasq and hostapd, using a USB 4G modem to connect to the internet. This is working as expected, I'm able to connect other devices to it and get network and internet access.

What I am trying to do is also share the USB internet through the ethernet port, so that I can hook up a RPi 3+ that way (WiFi on this board doesn't appear to work), both while being on the same network, ideally.

Is this possible?

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  • Tell us how you configure a hotspot using dnsmasq and hostapd. || Has the connected 4G USB device made a new interface on ifconfig?
    – M. Rostami
    Jan 21, 2020 at 1:21

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It is possible to forward traffic from the wired interface (eth0?) to the usb0 interface. Because you do not have provided any details about your network configuration I can only guess. You have to enable ip forwarding. There are several ways to do it: within /etc/sysctl.conf or by systemd-networkd or with /proc/sys/net/ipv4/ip_forward. Because you have already a working access point with uplink to your 4G modem, ip forwarding should already be enabled. Check if cat /proc/sys/net/ipv4/ip_forward returns 1. If so the wired connection could already work. Just try it.

If you are using a bridge instead of routing with enabled ip forwarding then you have to add the wired interface as additional slave to the bridge.

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