Are you seeing something like this?
"Whenever the phone has to switch from 4G to 3G or 2G, instead of transitioning, it drops the signal completely, there is no reception and it takes the device 1-3 minutes to regain it. If there is no 4G reception specifically, the phone has no service and it doesn’t come back until 4G coverage is regained. This occurs when there is an incoming call, an outgoing call or 4G coverage is gone due to physical/environmental reasons. Basically you cannot make or receive calls on the phone and you have no service if there is no 4G.
The issue's been reported from Germany and Belgium as well, there are several posts about it on the MIUI forums and XDA. Most likely some B20 related bug."
Found this on the /r/xiaomi subreddit...