Xiaomi MiWiFi connection sort of hangs after long idle

Using the snapshot OpenWRT, while using an Android 4.4.2 phone (LG) and if I just turn on (Home button) my phone and start using Chrome to visit any website, occasionally I can't visit any of them - "Site is not reachable" although my WiFi is connected to the Xiaomi MiWiFi router with no other active devices connected.

But I dont seem to have this problem with my Sony Experia Android 4.4.3 phone.

As control, both phones work without any issues with the stock firmware.

How could I solve or troubleshoot the root cause of this problem?