Finaly everything seems to work including LEDs with snapshot version :
https://downloads.openwrt.org/snapshots/targets/ramips/mt7621/openwrt-ramips-mt7621-xiaomi_mi-router-4a-gigabit-v2-squashfs-sysupgrade.bin
You can then install luci:
opkg update
opgk install luci
reboot
and voila
R4A-V2 version :
root@OpenWrt:~# lsmod | grep ^mt
mt76 51477 4 mt7615e,mt7615_common,mt7603e,mt76_connac_lib
mt76_connac_lib 40978 2 mt7615e,mt7615_common
mt7603e 40252 0
mt7615_common 70108 1 mt7615e
mt7615e 10277 0
R4A-V1 version
lsmod | grep ^mt
mt76 45120 4 mt76x2e,mt76x2_common,mt76x02_lib,mt7603e
mt7603e 38560 0
mt76x02_lib 40640 2 mt76x2e,mt76x2_common
mt76x2_common 11744 1 mt76x2e
mt76x2e 10208 0
The router was delivered with a firmware 2.30.28
I unbricked using 2.30.500