@Eirik 

Your post is very light on details:

1. What does "the internet bandwidth is limited" actually mean: what speeds are you getting in each of two houses? What is the speed of your Internet link from ISP?

2. What is the firmware version of Deco M9 Plus units?

3. Does Deco M9 Plus in house #2 report wired or wireless connection to Deco mesh?

4. How many Deco M9 Plus in house #2, if more than one: how they are connected to each other and do they all have slow speeds?

5. In Deco app, are there any alerts or warnings for M9 Plus in house #2? Something about link speed - latest versions of Deco M9 Plus firmware do warn if Ethernet link speed drops from gigabit to 100Mbps.

  @Alexandre. 

1. It might bee that limited bandwidth is not the correct term in this case. However, the user’s feedback is that they lose the internet connectivity In house nr 2 when connected to a stable Wi-Fi connection. If it is the overall bandwidth between house 2 and 1 who is bad is hard to tell. It might be that the cable between the house is damaged but on the other hand it strange since we have no errors in the network. I will measure the speed today.
2. I updated the firmware two days ago. I don’t have the number right now.
3. The M9 unit in house nr 2 report “Ethernet” and “Strong” signal. It is also a second antenna inn house nr 2 which report “Akseptab” on signal streght.
4. There are 2 antennas in house nr 2 they have a wireless connection.
5. No warnings!

  @Eirik 

Open "Update Deco" in Deco app, it'll show you current firmware version of your Deco units. It'll also show if different Deco units have different firmware versions.

If you updated firmware recently and problem appeared after, it would make sense to power off all Deco units at once and power them on after a minute or two, starting with Main Deco.

Firmware update supposed to restart Deco units, but for some reason such power off/power on proved to be helpful in addressing some issues after firmware upgrade.

If your M9 Plus reports Ethernet, it means it can use Ethernet link just fine. If you check Deco app page with Deco units from time to time and you always see Ethernet, it means Ethernet cable is good. It might run at 100Mbps, but that is different from losing connection at all.

According to TP-Link in lab tests with good quality Ethernet cable they managed to have Ethernet link between Deco units in up to 100m distance, but in real life 50m or less should be safer bet. 25m should be OK.

As long as it always says Ethernet for that M9 Plus, cable wiring issues are unlikely, the problem must be somewhere else.

Out of all your Deco nodes, and thanks for that picture, only Lager appears problematic. Any device that is near Lager Deco and connected to it will report strong WiFi signal, but actual Internet speed may be slow and Internet interruptions could happen for these devices.

If Lager is in house #2, see if people complaining of issues have devices commected to it. If they do, you will need to fix that by moving Lager Deco closer to Deco it is connected to.

Also, regardless, Lager Deco is not optimally placed. Even if it is not cause of the issue you described, you will need to fix it, too.

There is another possibility for house #2: WiFi interference. You might have device(s) in house #2 that occasionally cause WiFi interfernce: an old cordless phone, a microwave, etc.

There is less Interference on 5GHz band, so at least for the purpose of troubleshooting consider disabling 2.4GHz in Deco WiFi and see if situation improves:

If users connect to Guest Network, you'll need to disable 2.4GHz there.