It isn't necessary to build yourself. I used the pre-built initramfs image on the download server.
https://downloads.openwrt.org/releases/18.06.2/targets/ramips/mt7621/openwrt-18.06.2-ramips-mt7621-ubnt-erx-initramfs-kernel.bin
It isn't necessary to build yourself. I used the pre-built initramfs image on the download server.
https://downloads.openwrt.org/releases/18.06.2/targets/ramips/mt7621/openwrt-18.06.2-ramips-mt7621-ubnt-erx-initramfs-kernel.bin