WN3000RP-200UKS causes seemingly odd behaviour with WNDR4500 v2

I have retrieved the extender and updated to the latest firmware (feat. new UI) in the hope that the recent updates to both router & extender included a fix for this bug. However, the problem persists.

My router is exactly the same model as my mothers, and in tests the results are the same; after 12-24 hours network access via the extender is broken, and the routers ‘connected devices’ page reports that all MACs belonging to the extended network (02:0F:B5:xx:xx:xx) have no name, IP, or association with my wireless network(s) yet they are present/connected.

In fact, I noted on the routers ‘connected devices’ page that the extender (when seeming working correctly) has two MAC address 1) 02:0F:B5:xx:xx:xx and, 2) A6:2B:8C:xx:xx:xx [the Unique Identifiers (represented here by x’s) are the same for both MACs]. But unlike the first MAC, A6:2B:8C:xx:xx:xx has no name, IP, or association with any of my wireless networks. Is this normal behaviour?

I also wonder if any of the following router settings could be the culprit:

  Disable Port Scan and DoS Protection [Unchecked]
  Disable IGMP Proxying [Checked]
  MTU Size(in bytes) [1500]
  NAT Filtering [Secured]
  Disable SIP ALG {Unchecked]
  Advanced Wireless: use other operation mode [Unchecked]

How do I get this issue elevated? Is there a number or email for Netgear Technical Support?