Ran through each dtb using the latest CoreELEC-Amlogic-ng.arm-9.2-nightly_20200218-Generic.img build on the Droix X3 and here are the results using both the internal NIC and an external USB NIC.
Not working - Permanent black screen on the TV.
g12a_s905x2_2g_1gbit.dtb
g12a_s905x2_4g_1gbit.dtb
sm1_s905d3_khadas_vim3l.dtb
sm1_s905x3_2g_1gbit.dtb
sm1_s905x3_4g_1gbit.dtb
Not working - Continual Reboots.
g12b_a311d_khadas_vim3.dtb
g12b_s922x_odroid_n2.dtb
g12b_s922x_odroid_n2_dvb.dtb
g12b_s922x_ugoos_am6.dtb
g12b_s922x_ugoos_am6_rev_a.dtb
Not working - Continual Reboots & Corrupt boot Logo.
g12b_s922x_beelink_gt_king.dtb
g12b_s922x_beelink_gt_king_rev_a.dtb
CoreELEC Working but with no detected network or bluetooth devices (internal or external)
(Note: Both g12a device trees boot the initial partition formating screen and once complete require a hardware reboot before loading CoreELEC. The sm1 device trees do not require this extra step.)
This thread is not monitored for help and support, tou can not just try any device tree and hope for the best, the key is in the naming of them.
Based on the spec of the box the correct device tree is sm1_s905x3_4g.
If network is not working then it needs to be brought to our attention in a more appropriate thread, ie nightlies, you also havenāt said whether this is just wireless or wired or both that isnāt working.
As you have tried an external NIC as well it could be that the driver is missing but again these are all separate issues that need raising.
so you downloaded the correct ng build,you then copied the correct dtb from the dtb folder, renamed it to dtb.img & copied it to the root of the usb/sd card
you then held the reset button in, while plugging in the power lead. after a few seconds, you release the reset button?
if you did that perhaps you have not used the correct device tree(dtb)