Can you access internet from your wireless router directly?
Doubt you haven't configure range extender successfully.

Sure ,from my also tplink wireless modem router I can connect and have internet access easy,but when I connect through the extender I can't,I connect to WiFi but with no internet access. I tried all ways to connect with router,wps, manually from web ui with extender mode 1 and 2 and I always connect extender with router successfully, but from android tablet/phone using extender I connect to WiFi but no internet access.

I do have the same problem, I have small company that help to extend the wireless range, I decide to use tp-wa850re but i face this problem, its really disappointing!
Any solutions?

Well, I determined that the problem occurs when the TP-WA850RE is idle for some time, and the following are facts:

1. Connected clients cannot pickup a Valid IP Address.
2. The web interface is not accessible.
3. After disconnect/Connect the power from the device it begin to work again.

So its very clear that its an idle/sleep problem.
I searched for any configuration of idle/sleep mode in the settings but i couldn't find any.
But I'm pretty sure the problem not related to DHCP or Channel configuration.

Can any one from the Firm's team tell us how prevent the device from going to idle/sleep mode?

you guys did not post FW in use.
did you upgraded your AP to latest FW its http://www.tp-link.com/resources/software/TL-WA850RE_V1.20_130622.zip
and fixes are
1-Shortened WPS connection time and improved the success rate;
2-Changed WPS button policy to only work with the root device, not to clients;
3-Fixed a bug that sometimes TL-WA850RE cannot connect to some wireless networks after reboot;
4-Fixed a compatibility issue with some Samsung Smart TVs;
Fixed some vulnerabilities and improved security.
once with latest FW, do not use WPS to set yor 850 use a computer, if you are able configure method 2, see picture. use it if not method 1
post results this model has been a headache

Well danymarc,
I upgrade the firmware, it doesnt solve the problem and the device become more unstable!
Not that only but it affect the other computers which connected to router not to wa850re.

Did you test
Range extender method 1 and
Range extender method 2
With sane results
Justt checking is DHCP i disable
I posted on #6

if you are able configure method #2

if you get that behavior on your network means you cant use method #2 cause your main router does not support WDS
dont worry for that issue

become more unstable!

i get it in my network, when i set or do something wrong
example you are connecting to a router that has 192.168.0.1 and you set on your 850 AP 192.168.0.20 and you get that issue set 192.168.2.1 you are out of the subnet, you wont log into the 850 managment page wireless but you will have internet, at the end that is your main purpouse