I was able to make ethernet working with armbian by @SteeMan's advice. The key is to install the STOCK firmware. I didn't expect a custom android firmware like XTMOD can break ethernet in armbian. FYI, I'm using meson-g12a-sei510.dtb.
By the way, it seems like @balbes150is not happy (really) with even a mentioning of the name of the non-armbian who-know-what-it-is operating system. I understand it is done due to some strict rule armbian forum has, in order not to clutter the forum with irrelevant information. Or it simply could be because Mods/Admins don't like it. I was hoping my detailed description of my struggle could help someone encountering a similar issue specific to this box, but it doesn't seem so, apparently.
For those who come across this topic, long after it's closed, please keep in mind that the android rom installed in your box's internal storage may affect the functionality of armbian or the whatever non-armbian OS. If you find an usb or ethernet is not working, try installing a different android firmware other than the stock rom and test again.
Sorry for the hassle. Mods/Admins: you can close this topic now.