Diagnosis
The 5GHz band is likely getting interference from other devices or is suffering from a firmware bug that causes instability. It's a common gripe with this model.
How to Fix It
Log into the router's web interface (dlinkrouter.local). Go to Wireless Settings. For the 5GHz network, change the 'Control Channel' from 'Auto' to a fixed channel, like 36 or 149. Also, make sure 'Enable 20/40/80 MHz Coexistence' is checked. Save settings. This stops the router from constantly hopping channels, which can cause drops.
If That Didn't Work
Check for a firmware update in the Management section. D-Link has released updates that specifically address wireless stability. If you're already on the latest, try disabling 'Smart Connect' (the feature that combines 2.4GHz and 5GHz into one name). Create separate network names for each band.
Heads Up
Some older devices don't play nice with WiFi 6. If the drops only happen with one specific gadget, check its manufacturer for driver or OS updates.