first of all you should use
www.google.lt
as detection server.
In fact IP address is not always just one and sometimes some don't answer to ping requests.
Usually I setup router to ping not google but google's dns server 8.8.8.8 or opendns one 208.67.222.222
Furthermore ICMP Detection Mode = Monitor Traffic let the router suspend ping tests until there is traffic over WAN.
I usually set it to Ignore Traffic so it performs tests in any case.
Best regards
Marcom S.r.l.
via della Metallurgia, 11
37139 - Verona (VR)