I succeed installing Openwrt on R4CM with
I used openwrtInvasion to enable telnet working on firware 2.14.82 firmware of R4CM
acecilia/OpenWRTInvasion
A very good help came from Zorro
I used the Stable release of R4A
openwrt-19.07.3-ramips-mt76x8-xiaomi_mir4a-100m-squashfs-sysupgrade.bin
The steps I've followed
1/ have the routeur in routeur mode (not in AP)
2/ follow the video tuto of Zorro (except that I used Windows Powersheel with Debian bash instaed of his method of python on windows command)
3/ use the firmware above
Wan port is showing not connected but working