@pockypok 

What mode are you using the Deco's in, Router or AP?  I ask because the example you provided you said you are connecting a router to the 2nd unit.  I just want to make sure that was just an example and not a application.   The reason that could be a problem is the Deco's ethernet ports are WAN/LAN the 2nd unit could be detecting the router as a router and switching the port from a LAN to a WAN.  If a router is connected and the Deco's are in router mode try puting this 2nd router in an AP mode or removing it from the equation and see if the issue clears up.

Best Regards 

I'm confuse how can you set only deco second unit as AP ,

Since my modem at only brigde and i use first deco as pppoe to connect to the internet , I'm checking the APP but see only change the whole network to access point which i don't want :| , Any instuction how to config only 2nd Deco to AP , Thank you

@pockypok 

Sorry for the delay in a respone.  I did not get a notification of a reply.  We are working on this but please Tag me so i see the notification.  There may have been a mis cominication.  In you description of the problem you said  "pc / server / etc / router"  with that I thought you were stating there was a 2nd router as the Decos themselves are a router and that was what i was referign to make sure is in an AP mode.  

@pockypok I have the same issue, have you been able to resolve it?

@pockypok I have the same issue, have you been able to resolve it?

@arbel Same issue for me, any resolution? Looks like we are many in the forum that has the problem and no good answers from Tp link?