Iinternet disconnection/ DNS not respond.

Hi, I have install a Fortinet 100E it connect with wan port ip192.168.5.2 with ISP Device ip 192.168.5.1  and connect lan along with IP 172.31.6.0 and enable DHCP pool 172.31.6.51 to 172.31.6.250 , select system DNS IP at least 100 nodes, no any security apply every thing open.

all node connect with internet for 2 hours after that all user can't access to internet and can't ping to DNS 8.8.8.8(time out)

and that time all nodes ping to lAN , WAN, ISP router it result is successful 

could you please suggest me any solution to resolve this issue.