Vigor2862ac with latest firmware 3.9.6.1_BT

ESP8266 associated with 2.4GHz Wireless LAN:
Running example mDNS_Web_Server code.
Connected via serial cable and configured to provide debug output from MDNS responder.

Laptop associated with 5GHz Wireless LAN:
Running continuous ping script...
@echo off
:start
ping -n 1 esp8266.local
goto start

Works for up to a few minutes, then all MDNS broadcast forwarding between the 2.4GHz and 5GHz networks stop dead. This is as observed in Wireshark and also via serial debug output provided by EPS8266 MDNS responder. However webserver on ESP8266 is still contactable directly via IP address.

Disconnecting laptop from 5GHz and connecting to 2.4GHz and MDNS name resolution is restored (no reboot of ESP8266 required). So, when both clients are on the same Wireless LAN there are no connectivity issues.

Reboot of the ESP8266 (presumably because it forces a disaccociation and reassociation to the 2.4GHz wireless) will also temporarily restore name resolution from the 5GHz network. At least for a few minutes, then all packets are dropped again.

Any suggestions?

Update: Definitely looks like a Draytek bug to me.

Separate 2.4ghz router now plugged into one of the Draytek Ethernet ports and configured with different SSID.
ESP8266 associated with new SSID.
No problems with resolving the name of the ESP8266 from the Draytek 5ghz network in this configuration.

Support ticket raised with Draytek.

Please keep the forum updated with this.