If you need an IP address to be used for DMZ mode. You need to use the IP address of the Google Mesh. You may check the assigned IP address of the device when you access the TP-Link's Gateway and go to
Network Map > Under 'Basic' Tab there should be an option 'Wireless Clients' and 'Wired Clients'.
If I turn everything off, turn the VR1600V back on and wait for it to boot, and then turn on the Google Wifi main unit and wait for it to boot, then it will once again be OK until I try and add a second google unit onto it.
We'd like to clarify if you're trying to add another Wi-Fi point. If so, please confirm if the First Wi-Fi point works.
We highly recommend to check the step-by-step setup procedure available here.
OK, so it is early days yet, and at the moment all the points are together in one room, just to make setup a bit easier, but so far it has been working reliably for half an hour. I enabled DMZ mode on the VR1600V. At about the same time I replaced the little USB power supplies that came with the Google wifi system with better quality ones that I had already, because I wondered if a reboot cycle could be triggered by a faulty power supply. Anyway, whichever change was significant, so far it seems to be working. Hopefully that will continue.