Which version of macOS is this happening on?
Was the AVB connection stable in an earlier version of macOS?
Which Ethernet device are you using? Is it Apple's Thunderbolt-to-Gigabit Ethernet Adapter connected via Apple TB3-to-TB2 adapter?
Do you get stable, reliable Ethernet connection over this port (not AVB, but just regular Ethernet)? This can be tested by disabling Wi-Fi and just using wired Ethernet.
Last macOS Big Sur Update (11.6.x)
Apple TB3-to-TB2-to-GbEth (Yes... as you mentioned)
I haven't try to connect to the internet using this port.
My Wi-Fi/Bluetooth and built in ethernet ports are working (z390 designare) using the DMAR and VT-D method.
One thing I need to mention..... i did update the bios to F9j..... I was reading that this version is not to reliable? That is better to downgrade to F9i or F9g Beta, is this correct and could this be my problem right now?
z390 Designare
i9-9900k
gt 710
32gb ram
2x 1tb m.2
OC 7.6 (The one before the recent)