Despite the huge variety of possibilities in modern smartphones, they still continue to perform the function for which phones were initially created - mobile communications. The functionality of this function is provided by a SIM card inserted into the special slot (and even several).
However, it is not uncommon for new and already used smartphones to have problems with communication - Xiaomi does not see the SIM card. This problem has several causes, and their most accurate detection is very important in this case, since further actions depend on them.
These cases are good because most often the user can independently deal with this problem without resorting to the help of specialists.
Hardware nuances arise on new devices (most often this is a normal marriage) and on already used (perhaps the phone had time to fly somewhere or swim). Some nuances can be eliminated on their own, but more serious ones require knowledge of the case.
Does the SIM itself work? Perhaps she had been lying idle for a long time, and the operator turned it off. Contact the call center to solve this problem.
The sim card can be very old, the contacts are damaged or oxidized. Try to clean them with a cotton swab dipped in medical alcohol. Also check the contacts on the smartphone itself - if necessary, perform the same operation with them, using the minimum amount of alcohol to prevent it from swirling inside.
Is the card firmly in the slot? Replace the card from the operator or repair the slot by contacting a specialist.
Perhaps you have a version of the device, in which the modem does not support local mobile frequencies. This often happens when ordering a phone directly from China, which, accordingly, have a hardware configuration different from the export models. There's nothing you can do - negotiate with the seller.
Often there is a nuance on new devices - a normal marriage. Please contact the store where you bought the phone, let experts know about it (if the phone, of course, under warranty). Repair will not give up, of course, but with the guarantee is free. It is better to require a full replacement of the device - more likely that the problem will not make itself felt again.
If the above tips do not help to solve the problem, then, most likely, the matter is more serious. Phone stuffing may be damaged, some sort of postings or a loop may be damaged (this is usually manifested by some symptoms other than SIM card slots). It is advisable to contact an authorized service center for people who know how to fix these phones.