Problem Receiving MDNS broadcasts from Wireless Arduinos

[Teddyz]

My Arduino wifi-nodes (ESP8266) advertise their OTA-capabilities by broadcasting MDNS every 5 minutes.

They are 100% insecure, so I want them to be shielded from my local network when I do not want to change their software. So I have them on a guest network and disable Intranet access when not needed.

This, I think, worked fine on my AC68 router, but not on my AX88, both running Merlin.

BUT: When I put 1 Arduino on my local network, I only receive a few broadcasts after it has connected. Rebooting it's software does not help. Wifi is still connected, even if I reboot it).

AND When I put my Android phone on my guest network, I only got a few broadcasts after it connected, not thereafter. Usually I get a message every minute.

My PC is connected to the router directly via cable.
Guest network is 2.4GHz.
Local network is either 2.4 or 5, but my phone usually use 5GHz.

Below is a picture from one of the broadcasts that I received after putting the Arduino on my local network:

Any ideas?

 

[Teddyz]

I found that my Android phone reliably broadcasts 3 MDNS-messages every time I unlock it.
If connected to an Intranet-enabled guest-network on 5GHz, these messages are not received by my PC.
I think this "proves" that they are filtered in the RT-AX88U, except for a short period of time (some seconds) after connection to the guest WiFi.
I have UPNP disabled in the router. Can it be related? I would prefer to keep it disabled.

 

[Teddyz]

After more testing I found that I receive all the broadcasts from my Arduino when it is connected to my "main" 2.4GHz Wifi. Atleast after I disabled the 5GHz in the AX88 router.

The Arduinos I have on the guest networks are not seen with one exception, where one device sometimes shows up after the delay that happens when I save setting (e.g. enabling a guest network) in AX88.

I have a MESH from AX88 to a AC68. To the AC68 I have connected a N66 via cable and running it as AP. Arduinos that connect this way are not seen at my PC.

When I try to learn of how MDNS works, I see here https://en.wikipedia.org/wiki/Multicast_address that the destination IP address is for "the local subnetwork only". Am I violating that rule in my setup? All addresses are 192.168.1.x and I have only one DHCP-server. But guest networks and MESH and an AP perhaps breaks the chain?