I've just purchased two deco M5's and set the main one up as the router and the second one as an access point connected to my home ethernet network via a netgear gigabit unmanaged switch. 
 

All very easy to set up, but I'm finding after a few hours the router deco seems to drop the Ethernet connection completely, whilst the Wi-Fi remains working. The light on the main deco remains green. To start with none of my Ethernet connected POE devices (cctv etc) would work with the deco as the router either. 
 

I've been doing some troubleshooting with a tp-link technician and mysteriously the POE devices issue has resolved itself, however I'm still loosing the Ethernet connection after a few hours. 
 

just wondered if anyone else has come across this as supposedly I'm the only person to have experienced this issue. 
 

If I go back to my old set up and use my Apple time capsule as the router and set the two decos as wireless access points, the set up is very stable and they provide very good Wi-Fi coverage. I have no Ethernet issues in this configuration, so it's got to be something to do with the deco dropping the Ethernet when it's in router mode.