@BarisOz 

For troubleshooting purposes, it would help if you can clarify what do you have as a "modem:" is it just modem, or gateway? If you are not sure, give its brand and model number and I'll check that on Internet.

To make summary of your description:

Living room - good;

Bedroom - good;

Office room - issues, visible under high Ethernet link utilization.

--------------

If this summary is correct, the suspect becomes Ethernet cable which links Office room Deco with Living room Deco. Decos are unable to sustain gigabit link over it, and downgrade it to 100Mbps. 

Restarting Deco can bring link speed to default gigabit, but with high noise and packet loss over that cable Decos will eventually negotiate speed down to 100Mbps.

There are ways to test it further, but if you can just replace Ethernet cable between these two Decos, you should do that. Before doing that, unplug that Ethernet cable from both ends and plug it back, making sure it plugs tightly to Ethernet ports (does not feel loose). Sometimes that all it needs.