ASUS Router Error Code 21: How to Fix Wireguard Tunnel Offline

Easy 10-20 minutes Medium Severity Verified June 2026
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Error Code
21
Brand
ASUS Router
Product Type
networking
Severity
Medium
DIY Difficulty
Easy
Estimated Fix Time
10-20 minutes
Error code 21 on your ASUS router means the Wireguard VPN tunnel has dropped offline. This typically occurs due to a network issue, misconfiguration, or an expired key. Follow these steps to restore your secure VPN connection.
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How to Fix Error Code 21

  1. Check Wireguard Server Status

  2. Verify Wireguard Configuration

  3. Update Router Firmware

    Do not power off the router during firmware update.

  4. Restart the Wireguard Connection

  5. Check DNS and Firewall Settings

    Disabling firewall can expose your network. Re-enable after testing.
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When to Call a Professional

If the error persists after all steps, the issue may be with your internet service provider or the remote Wireguard server. Contact your VPN provider or ISP for support.

Frequently Asked Questions

What does ASUS router error 21 mean?
Error 21 indicates the Wireguard VPN tunnel is offline. This can happen due to server issues, incorrect configuration, or network problems.
How do I check my Wireguard configuration on ASUS router?
Log into the router admin panel, go to VPN > VPN Fusion or VPN Client, select Wireguard, and review the keys, endpoint, and allowed IPs.
Will rebooting my router fix error 21?
Often yes. A simple restart can re-establish the Wireguard tunnel if the issue was temporary.
Does updating firmware help with Wireguard issues?
Yes, firmware updates often fix bugs related to VPN stability and performance.
Can a firewall cause error 21?
Yes, if the router's firewall blocks Wireguard's port (default 51820/UDP), the tunnel can drop. Check firewall rules.

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