I have an Archer VR600 connected to internet via ISP fiber optic converted to Ethernet in WAN port. The router is set to Wireless router mode, no VLAN tagging.
 

Wireless clients works fine but wired clients loose internet access from time to time.

I tried to use different Ethernet cables and use the several LAN ports of the router but I get the same results. 
 

While this loose accrues, the wired clients still appear on the network (I can ping them) but they cannot access internet. At this time, wireless clients are able to access so it is not an ISP problem. 
 

What can I do in order to solve this issue?

Thanks.