You can use LEDE modifications made by obsy from eko.one.pl
He's maintaining OpenWRT and LEDE images for a number of different devices and keeps them up-to-date. He also provides superb support for anyone who needs help with setting up OpenWRT/LEDE on their devices.
Check out he's website: http://eko.one.pl/
He's forum: http://eko.one.pl/forum/ (most of the post there will be in polish but people has been writing in english before so don't get discouraged).
He's repositories: http://dl.eko.one.pl/
Here are LEDE images for Ubiquiti EdgeRouter X (those are WITHOUT LuCI):
http://dl.eko.one.pl/lede/17.01-SNAPSHOT/targets/ramips/mt7621/
lede-ramips-mt7621-ubnt-erx-initramfs-kernel.bin
lede-ramips-mt7621-ubnt-erx-squashfs-sysupgrade.tar
This file contains LuCI graphical interface:
http://dl.eko.one.pl/luci/lede/17.01-SNAPSHOT/targets/ramips/mt7621/
luci-ramips-mt7621-ubnt-erx-squashfs-sysupgrade.tar
Before you do anything though, please make sure you've made backup of all of your current partitions with original firmware, just in case something goes wrong.