pafos86, Yah?
In the stock firmware with which the device came
If you put on the first SIM 4G, then the second is always 3G
There was never a smart network on the drain.
On Miuipro 8.3.8 made a smart network, and you could set any settings for the second SIM card, at least 2G, at least 3G
And on Miuipro 8.3.15, a smart network has been shown and the second sim card was added for 2G (hello tele2-Moscow)