so I got openwrt working somehow. this is what I did:
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followed point's 1 and 2 from this guide to debrick the router https://forum.openwrt.org/t/xiaomi-r4ac-problems-installing-openwrt/82083?u=ry7itt5r. basically consist in downgrading the firmware to 2.18.28 using the MiWifi Repair Tool and then upgrading to 2.18.58 using the web interface (got 2.18.50 from here https://miuirom.org/miwifi/mi-router-4a)
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Followed hoodys guide but this time instead of flashing the openwrt build made by @WereCatf, flashed the firmware for the Xiaomi Mi Router 4A (MIR4A) 100M https://openwrt.org/toh/xiaomi/mi_router_4a_mir4a_100m#installation (that would be openwrt-21.02.3-ramips-mt76x8-xiaomi_mi-router-4a-100m-squashfs-sysupgrade.bin) like @diepeterpan suggeted.
after rebooting from the flashing proccess. the router enter a bootloop again and thought it was bricked, so I went (without hope) to do some chores and whe I came back the two leds were blue and could enter luci through 192.168.1.1. openwrt is working (for now everything seems to be working fine).
so I guess that all of this turned the router into the chinesse version of it???.
keep in mind that I've been messing with this the whole night (flashing and reflashing, unbricking etc). tried again and again, tried diferent guides. so I'm probably mixing up some details. if you are going to try this do it at your own risk.
thanks to everyone for the help