After hours of problem solving, I finally found out that when a WiFi is choosen, IPv4 and/or IPv6 is not set to ON/AUTO automatically.
In my case, the WiFi was semi-connected to my router, but because IPv4 and IPv6 was still on OFF, the REAL connection was not there.
Is it an idea to show a notification that if WiFi SSID is chosen, and IPv4 and UPv6 is set to OFF? Suggest to turn ON at least one of the 2.