DMZ IP address setting spontaneously changes on plusnet hub two

[#1]

Hi

I've got a plusnet hub two that was new this summer, I'm using another device as an openwrt router on my network as I am load balancing between the plusnet connection and another router on 4G.

The plusnet router has DMZ enabled and pointed at the external ip address of the openwrt. This is set as a 'user defined IPv4 device', i've attached pictures.

Everything works fine, until sooner or later the DMZ IP address spontaneously changes to that of another device. 

There doesn't seem to be any particular pattern or logic to the IP address that the setting changes to, I've seen various different IP addresses on my network - it always an IP address that is in use on the network.

Annoyingly, there is no record of the DMZ record being changed in the technical event log of the router.

To give you some more technical details, the network is completely flat, everything is on a 192.168.1.x ip address, DHCP is turned off on the plusnet hub and done elsewhere, the openwrt router has 192.168.1.2 as it's gateway, 192.168.1.3 as the external side for the 4g router, and 192.168.1.4 as the external side for the plusnet router.

I really don't need anyone critiquing the solution or telling me I shouldn't do things this way as it works fine when the DMZ setting is in place - it usually lasts at least 12 hours before changing, sometimes a couple of days.

Any ideas?

[#2]

How have you assigned the address 192.168.1.4 to the DMZ device?

I have played with this space recently (wanting to set up an open guest Wi-Fi service using a redundant router).  What I encountered is that the DHCP static address allocation pages were less than user friendly when seeking to assign a static address.  On several occasions I thought I had done it, but the device kept reappearing with its dynamic address.

Also, when you have successfully assigned the static address, it needs to be restarted! (But I am sure you know that!).

When the DMZ address changes "randomly" is that address the CURRENT LAN address for your other router?  I suspect that the DMZ "tracks" the IP address using that hardware MAC ID.

When I set up the DMZ, it was the name of the device which was showing in that box, not the IP address.

[#3]

Moderators Note

This topic has been moved from Broadband to My Router

[#4]

The IP address is set static on the openwrt router itself - as noted the DHCP is disabled on the plusnet hub - and this ip address is outside the distribution range for my DHCP server, although it is on the same subnet.

It does appear to be a reasonably new setting to use a 'user defined ip address' as DMZ, the previous plusnet hub doesn't appear to have this setting available, you must pick a device the hub has detected, so it may not be available on your hub if you are looking for the setting.