GoKawiilDynamic Frequency Selection explains why 5GHz Wi-Fi sometimes drops to 2.4GHz
Routers operating on DFS channels in the 5GHz band must perform a government-mandated radar scan, taking one to ten minutes depending on country, before activating. If radar is detected mid-session, the router exits within ten seconds and avoids that channel for 30 minutes; ASUS and FRITZ! routers handle this differently, with FRITZ! shifting devices to 2.4GHz and ASUS sometimes reducing channel width from 160MHz to 80MHz.