when i changed to run the current kernel 5.4.12 the ethernet would work ok, then while transfering over smb/samba to my windows 10 box the ethernet would totally drop, as in the light on my switch went out, not just dropping a few packets!!
The only thing that fixed it was to rever "back" to 5.3.11-imx6.
Note the change both up to the 5.4.12 and then back to 5.3.11 were performed via the armbian-config tool.
(the system is running headless, and the log file does include a boot of "Linux version 5.3.11-imx6" as well as the working 5.3.11