WiFi problem on XIAOMI?

Question

Android

Good day to all!
In general, I ran into a problem - I bought a dual-band Wi-Fi router MERCUSYS AC12 (AC1200) (Mercury MAC 1200). There is a XIAOMI Redmi 5A phone. There are no problems with connecting to a 2.4GHz network, but there is a problem with 5GHz. The phone does not see this network and cannot connect, although it is enabled in the router settings. Where and what could be the problem and how to solve it? Help me please!