Hi!
As I said, it is the 2.0.376, that you can find here: http://cdn.cnbj1.fds.api.mi-img.com/xiaoqiang/rom/r2100/miwifi_r2100_firmware_64288_2.0.376.bin
You can downgrade it using the stock's interface, through the upgrade option and then select the .bin file manually, or you can use the MiWifiRepairTool.
Once you have downgraded the system (make sure that the interface shows the 2.0.376 version), I think that you have to unplug and plug the router, because I tried without doing it and I was not able to install OpenWRT until I did it.
Hope it helps, if you have any questions do not hesitate and contact again.
Good luck.