Diagnosis
This is a classic symptom of the router's wireless driver or firmware getting confused, often related to specific device connections or channel interference. The 5GHz radio is essentially crashing and restarting, which takes it offline for a minute.
How to Fix It
First, log into the router's web interface and update to the latest official firmware if you haven't. After updating, do a full reset: hold the reset button for about 10 seconds until the router reboots, then reconfigure from scratch (don't restore a saved config). During setup, try manually setting the 5GHz control channel to something like 36 or 149, and avoid DFS channels (like 52-144) which can cause the radio to shut off if it detects radar.
If That Didn't Work
There might be a specific device on your network causing the crash. In the router's Wireless -> Professional settings for the 5GHz band, try disabling "Universal Beamforming" and "MU-MIMO" as a test. If the problem stops, you can try re-enabling them one at a time to find the culprit.
Heads Up
If you're using the same SSID (network name) for both 2.4GHz and 5GHz, some older devices can cause weird handoff issues. Consider giving your 5GHz network a slightly different name during troubleshooting to see if the problem persists.