How to share a 4G data connection over wired Ethernet? I understand that Android is principally an wired-Ethernet client.  i.e. it requests and receives a DHCP address.  The internet is on the other side of the Ethernet cable.
Does any version of Android have the capability of running its wired-Ethernet in an &quot;access-point&quot;, &quot;tethering&quot; or &quot;server&quot; mode, where Android's Ethernet provides a DHCP server and NATs Ethernet traffic out through its 4G radio, to the internet?
While the feature has never been part of stock Android (not sure if some custom ROM provides this), it's possible on rooted devices.
I recently answered to How to make Ethernet work on Android over OTG? which may help you make Ethernet work on your device. Once done, there are two more things required:

Run a DHCP server (we already have dnsmasq on device up to Android 9, or you may get a static binary)
Share internet (by doing IP forwarding and NATing)

Both of these steps are explained in my answer to How to create a WiFi hotspot from the terminal? and the answers linked there. Just ignore the steps related to WiFi and pick the relevant parts.