@David-TP Thank you for replying.

My ISP is Telia (Norway). The cable modem is a Sagemcom 3890v3, software version FAST3890V3_GET-SIP_sw05.76.9.7v-6.

I tried changing the main Deco, which did not help.

Since my previous post I have replaced the cable modem twice (with new modems which are the same model).

The cable modems have also been reset. The problem persists. 

I need Internet access in places that are not covered by the cable modem, so what I am doing currently is that I have a WiFi repeater with a cabled option connected to the WiFi of the cable modem. The main Deco is cabled to the repeater, so I have full coverage through my mesh setup.

This is not ideal, of course, and the maximum speed in the Deco network is 20% of what I'd get with the Deco cabled to the modem, but it has been stable for a day and a half. 

  @David-TP I have not been able to solve the problem, really. Wiring the X90 to the modem "kills" the ethernet ports of the modem after a while when they are connected directly, and the modem needs to be restarted to fix it.

Now I have an old Netgear router placed between the Deco and the modem, so I have a wired connection and it is mostly stable, but it is annoying to have an extra device running which should not be needed.