@marcus_w 

there is a suspicion that smart dhcp can be turned on on deco in access point mode

In the screenshot in the settings I highlighted this function.

if it is present, turn it off and test

if this function is not there, try to switch the main deco to router mode and also test

  @Godlike66 hi! thanks for the reply!

Before the last time the problem occurs, i have disabled the Smart DHCP for test... but stills occurs again a day or two later. The problem occurs both when its in Router Mode and AP Mode. Its better i keep the Smart DCHP off? thanks

  @marcus_w 

Smart dhcp must be turned off. It is a very parasitic function.

and of course, if problems arise even without it, it would be good to try replacing the cable between the provider's router and deco

well, and just in case, update deco itself to the latest firmware version, if this has not been done yet.

Let's see, maybe someone else will have other ideas

  @marcus_w 

I would suggest to simplify network layout by having Deco mesh in Router mode and removing Archer C5 in House 2. Main Deco should then be connected to Archer C5 in House 1.

  @Alexandre. i was thinking to do that, will try this. Thanks!

  @Alexandre. One question, the Archer c5 its already doing the work of Router, so i shouldn't keep the Decos in Acess Point mode? Or the correct is Router Mode?

  @marcus_w 

I would suggest to start with Deco mesh in its default Router mode. Conceptually, that would make House 2 like connected to the Internet. House 2 will not depend on House 1 for network management.

Because, it is not clear what is going wrong, having something simpler and running independently from each other is the first step.