@Adrz In Deco app, open More/Advanced and check if you have Smart DHCP setting there. If you do, is it set to Enabled or Disabled?

@ideron IP range 192.168.1.X is for when Deco mesh is in Router mode. Check Operation Mode in Deco app, under More/Advanced: maybe firmware update or accidental finger slip while in Deco app put Deco...

@JoMiJe Check if this applies to your situation: How to extend guest network when there is a switch between Decos

@P1ur Solution for this, and other similar issues is to configure Deco mesh to run in Access point mode: Switching a Deco to Access Point (AP) Mode That will make all devices in your home network run...

@Cahagg01 If this is what Test Internet Speed shows you in Deco app, it still means link between Main Deco and modem/router is 100Mbps. Check your modem/router documentation, most do show negotiated...

One more thing: have you checked Signal in Satellite Deco, and what it says?