Just got the router as it was sold to me as a "3G".
Apparently Xiaomi did cut corners and this is a rebranded "4A Gigabit Edition" and there's some advanced discussion about it here: Xiaomi Mi Router 4A Gigabit Edition (R4AG/R4A Gigabit): fully supported but requires overwriting SPI flash with programmer So basically there's no openwrt yet.
The manufacture date on mine is "06.2019" so I assume it's unsafe to buy the 3G from china now. Be careful what you buy!
Edit: Managed to get a 50% refund from AliExpress, so don't be afraid to open a dispute!