Thanks to the exploit from @acecilia, I successfully root login into the R4CM (Factory firmware version: 2.14.81) and installed the official 19.07.2 firmware for the Xiaomi R4AC without using the SPI flash programmer.
It all works well except the WAN port. It looks like the WAN port (eth0.2?) can't get the IP address from the upstream. BTW, the LAN port and 2.4G WiFi works fine.
Now, the only thing I need is a good working firmware for R4CM 
@xTomax How did you build the image for the R4CM? What's the patch you used? Any special settings about the WAN port?
I would like to try to build the R4CM image myself.
Thanks a million!
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