Thanks a lot for the prompt answer,
Currently I can only access remotely via the VPN (the device is far from my home) but everything seems to be OK as I have a device connected to the LAN port and this device can access Internet and send emails through the GL-AR300M.
My configuration is also OK with DNS set to 8.8.8.8
Ping to 8.8.8.8 is working
ping 8.8.8.8
PING 8.8.8.8 (8.8.8.8): 56 data bytes
64 bytes from 8.8.8.8: seq=0 ttl=116 time=32.012 ms
64 bytes from 8.8.8.8: seq=1 ttl=116 time=42.346 ms
ping google.com not working
root@GL-AR300M:~# ping www.google.com
ping: bad address ‘www.google.com’
I tried different options for the customs DNS, does not work
For the VPN I use OpenVPN