Hi,
Into wiki page about the ubiquiti edgerouter X, at the section installation, it seems be an error with the squashfs sysupgrade version: always 19.07.0 and not 19.07.2.
(but in this download page, the good version exists)
$ ftp https://downloads.openwrt.org/releases/19.07.2/targets/ramips/mt7621/openwrt-19.07.2-ramips-mt7621-ubnt-erx-squashfs-sysupgrade.bin
$ sha256 -C openwrt-19.07.2.sha256sum openwrt-19.07.2-ramips-mt7621-ubnt-erx-squashfs-sysupgrade.bin
(SHA256) openwrt-19.07.2-ramips-mt7621-ubnt-erx-squashfs-sysupgrade.bin: OK