I have a one year old Deco M4 (not sure where to check the hardware version).  When I set it up I followed the apps simple set up instructions.  I have three units set up in router mode, with the main unit connected via ethernet to the port on my ISPs modem/router combo.  The ISP modem/router also has a separate wireless network that wirelessly serves the cable tv set top boxes and requires to be set to router mode.  

So both the Deco and the ISP modem are functioning as routers, both serving DHCP, which I believe is not technically correct and can lead to issues with double DNS? I have not noticed any performance issues, however I am wondering if I should be switching the Deco to access point mode? 

Any skilled advise on this would be appreciated!