Wifi extender EX3700 loses connections to devices

My devices keep losing wifi connection to a wifi extender EX3700. Wifi works without issues once a device is connected. But after a period of inactivity of about 10 mins or more, the connection stops working, whereas it may still work fine for another connected device. 

Network tests show that download still works, however most of pings and upload requests hang. Some upload packages still get through several times a minute, which manifests in problems like video streaming freezing for half a minute, or having to press a button in the browser several times before it works. Simple reconnection (e.g. switching wifi on the affected device off and on) fixes the problem for some time. This is extremely frustrating. 

This affects all Android devices in the house (phones, tablets), but rarely PCs - I guess because PCs don't usually become completely inactive for several minutes. 

It seems that the problem started to happen a couple of months after i've bought the wifi extender. But I'm not sure, we may just have been lucky not to notice it initially. I've just updated it to the latest firmware in a hope that the problem is fixed, but it is still there. 

The wifi router whose internet connection the extender extends is a Sky Broadband hub (UK). 

Did anyone else face such issues? I can't believe it's only me. Any help would be much appreciated. 

Thanks