Why would wifi extender actually interfere with android wifi

[#1]

Recently posted about wifi probs and pointed finger at Technicolour router,
But turns out culprit is TP-Link TL-WA850RE 300Mbps Universal Wall Plug Range Extender/ Wi-Fi Booster
I bought this to extend wifi range. But, instead when it is switched on my two android devices (samsung phone and tablet) do not connect to the internet.  THey can connect to my home network but cant see the internet and report no connection. They do not have any problem on wifi networks outside my home!
My wired connections and windows 7 netbook don't experience this.
Switch off and all devices fine????
Any ideas
Trevor

[#2]

All I can think of is to download WiFi Analyser to your phone and make sure that the extenders are in fact on the same SSID as the main router.
When I first installed a TPLink kit they linked to each other but not to my existing Netgear powerlines.
Started from scratch by only installing the TPLink ones first then using WPS to link to the router then added on the Netgear ones - all now OK.
WiFi Analyser will show a top level bar with all linked devices as a sub bar.

[#3]

Quote

Any ideas

Yes, sounds like the TPLink isn't configured correctly. It SHOULD autoconfigure but it sounds like it's got it's DHCP server turned ON. It's then giving out the wrong gateway address to it's wireless clients. Check the status of the TPLINK as shown on page 10 of the manual , it's DHCP server should(probably) be Auto(off).