You might try setting up the DAP using the wizard to set up the extender mode of the DAP. You can review this in the User Manual on page 18 and there is more info on page 33 for Extender mode settings.
That's essentially what I did, although I entered the security key manually (page 28). In the advanced network settings (page 29), I also tried both dynamic (DHCP) and static configurations. I had the same problem regardless.
What wireless modes are you using on the main host router? G and N or single modes?
N single mode for both 2.4 GHz and 5 GHz bands.
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One possible area to look at is the way that the dual band extension is implemented. You can only choose one channel to act as the link to the main router (page 27). In my case, I chose the 5 GHz channel (SSID5) which I have on channel 36.
The 5 GHz channel is extended in-band -- i.e. both the main router (SSID5) and the DAP (SSID5-EXT) use this channel -- which is pretty standard for a simple, transparent range extender.
However, the DAP does not extend the 2.4 GHz channel in-band -- it chose an "unoccupied" (low interference) channel for the 2.4 GHz band (channel 11 for SSID2.4-EXT instead of the channel 3 that my router is using for SSID2.4) and appears to backhaul the 2.4 GHz traffic on channel 11 over the 5 GHz channel 36 as well.
So, in the 2.4 GHz band, it looks more like another AP than an extended channel ... which are also the symptoms of my problem.
The DAP-1520 looks like it is based on software for a full AP that has been "dumbed down" via configuration options to an extender. I guess it is getting its roles confused somewhere :-}
I would like to be able to disable the 2.4 GHz but I am told that can't be done. I am also not able to choose the 2.4 GHz channel used by the DAP for SSID2.4-EXT; it selected this channel (11) automatically.