If anyone want to investigate this :
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Downloading official firmware is a good start : https://miuirom.org/miwifi/xiaomi-router-be3600-2-5g
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Some quick search shows that it's based on OpenWrt 18.06
DISTRIB_ID='OpenWrt'
DISTRIB_RELEASE='18.06-SNAPSHOT'
DISTRIB_REVISION='unknown'
DISTRIB_TARGET='ipq53xx/ipq53xx_32'
DISTRIB_ARCH='arm_cortex-a7_neon-vfpv4'
DISTRIB_DESCRIPTION='OpenWrt 18.06-SNAPSHOT unknown'
DISTRIB_TAINTS='no-all busybox override'