Unfortunately I have a similar problem. CoreElec Works OK but the speed of the wired connection is only:
ethtool eth0 | grep -i speed
Speed: 100Mb / s
Is my purchased (X96max+ 4/64) only 100Mb / s?
I used the sm1_s905x3_4g_1gbit.dtb file and the CE starts up correctly but only 100Mb / s …