This issue is fixed in upstream u-boot by commit a246e21 ("arm64: dts: meson: odroidc2: readd PHY reset properties"), which is included in u-boot v2021.07-rc1. The regression was caused by the backport of a kernel commit that had change a reset timing too aggressively, and first went into u-boot v2020.07. See upstream discussion, openSUSE bug 1187095.
Had same problem, can confirm that rollback to u-boot 2020.04 helps.
Also, picking c183c406c4321 from linux into u-boot where applicable works for me too,
at least on 2021.04.
I just wanted to write an update on this issue as I've ran into it also, but I figured out a way to make it work.
My board was in a "broken" state where the network card wasn't working with any of the latest versions of Armbian using a newer u-boot like 21.04-rc3.
I compiled my own versions to test the latest edge and using u-boot 21.04-rc4, etc. but nothing worked.
I got it working by accident and I wasn't sure how but then I flashed the sdcard again with a different version and got into the broken state again.
This lead me down an evening of frantically testing a bunch of things and that's when I discovered a way to get out of this broken state with a newer
version of u-boot (21.04-rc or later).
Grab the latest stable image which uses u-boot 21.04-rc3, flash it to an sdcard and boot the board and wait 5 min or something to make sure the board has actually booted (I'm running headless so no monitor connected).
Once you're sure the board has booted, unplug the network cable and reconnect it. Then wait for a couple of minutes and then power cycle the board and that made the network card work again for me every time I
got into this broken state.
Hope it helps someone else and if not I guess that something else made my network card work again.
zimme got a reaction from lanefu in Odroid C2 : no eth0 with latest image