I had a Mi(XiaoMi) Note pro in the past. Redmi and XiaoMi note share the same UI system. And this situation once happened on me. First check that if your XiaoMi UI is the formal UI or official Experienced UI.
If your UI system is an official Experienced UI, I suggest that you can send your phone to the Store to have a check
If you UI system is the formal one, press *#*#6484#*#* then you can check your sim card is good or not. If it shows that it doesn't works well than it's necessary to go to a Xiaomi Store to ask for help. The *#*#6484#*#*is an main simple way to check sim card works good or not.
I actually have some other ways to help, but I don't want to tell you all because that in China the workers in XiaoMi Store is helpful and I once had a try. BE CAREFUL when you're trying to fix some problems by yourself. Some of the good services are actually ONLY been developed very well in CHINA.