If the 5GHz Frequency Band is unstable:

 

• Change a Wi-Fi channel: Try installing the Wifi Analyzer application on an Android mobile device to scan the surrounding Wi-Fi signals to avoid congested channels (please refer to this video for more information).
• Avoid using DFS (Dynamic Frequency Selection) channels: Avoid channels 52-144 and choose between channels 34-48 or 149-161 instead. As DFS channels can automatically switch to other channels with lower interference, they may cause a brief disconnection during the switch.

 

 

• Make sure your device supports 5GHz Wi-Fi: Some client devices do not support certain 5GHz channels. Please refer to the specifications to check the limitations.
• Switch to another Wi-Fi bandwidth: If your bandwidth is currently fixed at (80+80)MHz, try switching to 20/40/80/160MHz for better compatibility.
• Move closer to your Synology Router: 5GHz networks are more suitable for open spaces. As they have relatively poor diffraction, obstacles (such as cement walls, wooden walls, iron, glass, etc.) between your device and router can greatly reduce the transmission speeds. Please stay as close to your router as possible when testing Internet speeds.

thank's for return

I use another PC the wifi works fine.

I can be connected in 5ghz but not longer. 

And I have this event message : Le miniport Killer Wireless-n/a/ac 1435 Wireless Network Adapter, {508c1606-fcb6-4d73-b12a-c9047b23db2a}, a eu l’événement Fatal error: The miniport has detected an internal error