AX88179 ethernet adapter keeps dropping connection

Solution 1:

The fact that WiFi is working fine, tells you that the issue isn’t related to your OS. Especially, your firewall can’t be an issue because a firewall won’t allow DHCP traffic then suddenly block it while simultaneously continuing to allow it for another interface. The tcpdump is telling you that the DHCP client that requests the IP from the server is working fine. The “oui unknown” means that the Organizational Unique Identifier (first 3 octets of the MAC address) can’t be found. Which means it can’t identify the chipset.

That leads us to....

This chip, made by ASIX Corp is an off brand, low cost chip used in many discount peripherals also made by off brand and discount manufacturers. In fact, a number of products based on this chip like this Plugable USB to Ethernet not only doesn’t list macOS as supported, it actually says not compatible with Catalina.

The problem then is the adapter and not your Mac. Steer clear of off-brand products and look for adapters that use quality chipsets like Anker USB Ethernet Adapters (I personally use this brand due to their reliability and warranty; 18 months). Generically speaking, look for chipsets from known, market leaders like Intel, Broadcom, Qualcom, Texas Instruments, etc. Chances are, the WiFi adapter used by Apple for its AirPort adapter is a Broadcom.