DIAGNOSIS The 5GHz band is likely suffering from interference or a channel conflict. It's more sensitive to things like neighboring networks, cordless phones, or even radar systems if you live near an airport (that's the DFS feature kicking in).
FIX Log into the router's admin page and go to Wireless > Professional for the 5GHz band. 1) Look for 'Control Channel'. Set it from 'Auto' to a specific channel. Try 36, 40, 44, or 48 (the non-DFS channels). 2) Also, set 'Bandwidth' to 80 MHz if it's on 160 MHz, as 160 is more prone to dropouts. Apply the settings. This forces the router to stick to a clear lane of traffic.
IF THAT DIDN'T WORK Check for overheating. Feel the top of the router. If it's very hot to the touch, it might be throttling the 5GHz radio. Move it to a more open, ventilated spot, away from other electronics or direct sun.
HEADS UP Some older devices have trouble with WPA3 security. If you have it enabled (Wireless > General), try setting 'Authentication Method' to WPA2-Personal for the 5GHz band to see if stability improves for all your gear.