Only snapshot version of openwrt available for mi ac2100. (ie. wait for openwrt 20 for stable release)
You'll have to read through the thread to decide if it is what you want.
https://forum.openwrt.org/t/new-xiaomi-router-ac2100/48101
Word of warning. There is the Mi AC2100 and Redmi AC2100.
Similarly, snapshot also available for Xiaomi AC1200 4A/3Gv2 gigabit router.
https://forum.openwrt.org/t/xiaomi-mi-router-4a-gigabit-edition-r4ag-r4a-gigabit-fully-supported-and-flashable-with-openwrtinvasion/36685
Snapshots are moving/using linux 5 kernel, and there are problems.
19.07.x uses kernel 4.14.