@David-TP 

Hi David,

The topology I put up is the following

ISPmodem/router---UTP--- main DECO X20 (kamer 2R)
                            \---UTP---satelite #1 DECO X20  (kamer 3F)

                           ( \---UTP---satelite #2 DECO X20 (kamer 2F) ) I add this one but after doimng that ( and maybe updating the firm ware sat#1 and #2 refused to connect through ethernet backhaul, then I left sat#2 unplugged)

This is a student house with 4 floors and 2 rooms per floor, every room has one UTP running to the location where the ISPmodem/router is on the groundfloor.

The ISPmodem/router is on the ground floor, kept giving good signal to the users there and after I installed the two deco's in september everybody was very happy for 5 months, till a month ago, the users on the upper floors started to complain about a very bad internet connection. One time it seemed to improve just by rebooting the main deco, but the complaints came back. So I decided to add a third deco in the room of the student with most complaints. but that installation revealed that the ethernet backhaul did not come again , not to sat #1 and not to sat#2.

This morning I ran a network optimisation on from the deco app, the ethernet backhaul came back to sat #1, ( not to sat#2, because I left that unplugged)

the ISPmodem/router is from proximus/mobile vikings, as I am not thee now I cannot check type.

I readin the meanwhile that this cofiguration is not adviced, but it worked fine for 5 months, and easy and conveniet.

anyway, even if I put the deco's in AP-mode, de deco's have the following IP adres 192.168.129.0 ( zero?)  aand sat #1 has 192.168.129.1, wereas the router has 192.168.128.1

I include a screenshot of the ISP

As I was happy about the deco's I bought a second box and kept it at home,

So I build the same configuration from my ISp/modem from telenet, I succeeded but my network and internet access got very slow, I unplugged alle the deco's again and this mornig the network at home was running like before.

  @ARGamingPK1 

Not sure? The backhaul came up after I ran the networkoptimisation from the Deco app. with the mask at 255.255.254.0

while the deco's had ip adresses 192.168.119.0 ( main deco) and 192.168.119.1 ( sat#1).

Your remark about subnet mask, made me study this and a subnet mask of 255.255.254.0 allows IP adresses 192.168.118.1 till 192.186.119.254 forming 1 network, with network IP adress 192.168.118.0, router 192.168.118.1 and broadcast adress 192.168.119.255, so nothing wrong then by having 192.186.119.0 as an ip adress. I never saw that because the subnetmask in all my cases before was 255.255.255.0.

So that is sorted,

but I see now on my app ,that the whole network is down today, no clue what happened, I will have to go there physically.

  @David-TP 

even further I reproduced the problem today again: if you connect the sats tot the modem/router, ethernet backhaul starts up.

But it brings the performance of the entire network down, not sure why. I can see reasons. Indeed in support documents I read that tp-link says you may not install the system like that. ( see the red cross on https://www.tp-link.com/en/support/faq/1794/)

but my question remains why did it work then fine for 5 monthsand the text in this post indicates it should work

https://www.tp-link.com/nl-be/support/faq/1432/ it should be ok in accespoint mode

and

this also means I bought the wrong product, given how my UTP cables are installed in the house, and it seems i am not the only one who comes to that conclusion.

  @ARGamingPK1 

Ok interesting, then What switch should I buy to save my investment in 6 Deco X20's and make sure it remains compatibel.

Is there not a Deco X?? that I can use as main deco and which has more than 2 ethernet ports, so that I can position that next to my ISPmodem/router, where all my UTP cables end up. these UTP cables come from the places where I need an extra AP.

In fact this switch should hav ebeen included in the package I realise now. as the main deco does not have enough ethernet ports, so you need a switch that is compatibel with the deco backhaul system

Then what could the reason be that my router did the job as a router and a backhaulswitch for some time very well? Maybe the router or the  deco's got a firmware update that made them incompatibel to keep on doing the job of switch and router together, whereas they were able to do that before??

I copy the topology again that worked fine for 5 months, which made me buy a second box with 3 Deco x20's

ISPmodem/router---UTP--- main DECO X20 (kamer 2R)
                            \---UTP---satelite #1 DECO X20  (kamer 3F)

also in some info of TP-link I read this can be done in other info it says it cannot be done.

depending on the router? and the firmwares? maybe? 

then what shoulds I buy to make sure it keeps on working throug all future firmware updates,....