@LuisFe
There are two common possibilities: Ethernet cable and Deco app configuration settings.
We can start with Ethernet cables, because if you have close to 100Mbps it often means gigabit link can't be negotiated over the cable. The occasional bad luck is to have Deco app configuration causing exact same drop of speed, but we can leave bad luck for later.
You have two Ethernet cables, and no simple means from Deco to figure what speed is negotiated over them. Still, there are ways.
Have this setup: LAPTOP ----- CAT6 cable ------- DECO ------ CAT6 cable ------- MODEM
Confirm the speed is still 100Mbps, using two different speed test apps: Ookla Speed Test app you can download and install on laptop, and web site FAST.com
If speed is 100Mbps with both speed test apps, do the following to narrow where the issue could be:
1. For cable between Deco and modem.
When you open Deco app, do you see "Abnormal WAN port speed" error message in it:
2. For cable between laptop and Deco.
Check what wired speed is negotiated at laptop's LAN network adapter, see How to check the associated/link speed on a wired client