Ethernet and PXE booting is tested on mainline u-boot, on current Rockchip u-boot 2017, it’s broken. We are moving to the mainline u-boot now. There are some features we want to port to mainline u-boot. We also implemented the NVMe driver on the mainline u-boot now. It works now booting NVMe from u-boot, which is not yet implemented on any other rk3399 boards.
If you can wait, please wait for our update, or you can build the mainline u-boot now, with the rk3399-evb config and dtb, it works on rock pi 4.