What can be the reason for "internet may not be available" though the IP address was got Only one device with Android (SM-G390F, Android 7.0, G390FXXU2ARA1, 3.18.14-12962328) at my home stopped connecting to Internet through my WiFi router with the message "internet may not be available" and an exclamation mark on the WiFi sign. (Whereas older non-updated Android devices, GNU/Linux and iOS devices work.)

If I examine the current network configuration with ip a in a Terminal in the device, it has got the correct IP address, but pinging the router (ping 192.162.1.1) looses all pings... (ip ro showed a route to this network, but no default route -- but that shouldn't have affected the pinging of the router anyway.)

The WiFi router is an ASUS one, and for this device, in the log, it prints several times (not more than 10; the repetition is only for this device):

udhcpd: sending OFFER of 192.168.1.8


and then:

udhcpd: sending ACK to 192.168.1.8


The DNS servers from the DHCP configuration seem to be working well (as tested from other connected devices).

Changing the advanced settings from DHCP to manual does not help. And it is quite strange that this device used to work with this WiFi router all the time before a couple of weeks ago... Nothing has been changed.
Strangely, this advice from 2016-09-14 concerning the WiFi router configuration helped:


  Inside the router configuration (e.g., accessing the IP 192.168.1.1), change the "Group key update" value from 0 to 30 seconds, under the "Wireless > Security" configuration, right below the WPA-PSK/WPA2-PSK item.
  
  Router: TP-LINK TL-WR740N / TL-WR740ND.


(My router model is different.)

I'm a bit worried that 30 sec as the group rekeying interval is the minimal possible, whereas the recommended value is 3600 sec (1 hour), as I've read on the web.

But for now, I haven't yet tried to experiment with since it began working. Also, I have left the static manual IP configuration instead of DHCP on the Android device (matching the reserved IP address for this MAC address manually in the router settings), although this alone didn't make it work. (But I'm not changing this, unfortunately for the experiment, because I don't want to break the current working state.)