Arduino -Raspberry Pi tandem, sharing an internet connection from a SIM800 mod

Hi I have an Arduino -Raspberry Pi tandem, connected together over serial GPIO.

The Raspberry Pi gets an Internet connection over a SIM800 and +++AT commands.
What could have been the solution to tether this Internet connection to the arduino as well?
ideally forwarding IEEE 802.3 frames in a transparent way over the serial connection.
Tt does not need to be fast.