Sorry for necroposting, but I was still monitoring this thread, as I have same problem with my Odroid C2.
I simply want to note, that I tested last image with Ubuntu 22.04 - still no ethernet in eth0, even if I install linux-u-boot-odroidc2-current=21.02.3 (and "flashed" o bootloader) or even with 2019.07 U-Boot.
I also tested multiple archived images (Armbian_20.02.8_Odroidc2_bionic_current_5.4.28_desktop, Armbian_20.05.1_Odroidc2_focal_current_5.6.15_desktop, Armbian_21.02.3_Odroidc2_focal_current_5.10.21_xfce_desktop, Armbian_22.05.4_Odroidc2_jammy_current_5.10.123_xfce_desktop), on every image I flashed U-Boot 2019.07 from Armbian_5.95_Odroidc2_Ubuntu_bionic_next_4.19.69_desktop - and still, no ethernet in eth0.
So, for me, it's looks like ethernet is still not working with 5.* kernel; with Armbian_5.95_Odroidc2_Ubuntu_bionic_next_4.19.69_desktop my eth0 works normally (as already was described somewhere above).
[Well, still, Armbian_5.95 have Ubuntu 18.04 and kernel 4.19.69, not "SO old", cause Hardkernel images have 3.16 kernel maximum]
Also point: my board revision is ODROID C2 REV0.2 20171114 . Looks like 2017 revision boards have this problems more "often".
P.s. - I know that Odroid C2 is EOL enough time ago
