Long story short:
I bought a Xiaomi R4AC 100M router, which had the 3.0.5 global stable version firmware, which was always bricking when trying to install OpenWrt. I followed the steps below to get it to function:
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Downgrade to chinese firmware 2.18.28, using MiWifi Repair Tool.
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Update to 2.18.58.bin manually through the xiaomi web interface
(Procedures can be found here Xiaomi Mi Router 4A Gigabit Edition (R4AG/R4A Gigabit) -- fully supported and flashable with OpenWRTInvasion Thanks to @beitabeet). -
Then followed up with @hoddy tutorial to install OpenWRT. (Thanks for the time you had spent with my questions).
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In case you brick the router, @hoddy has an excelent tutorial on how to debrick it as well.
Case closed and solved