Hi @Jaminjam . The Google Nest is configured by phone app, so there's no user guide to read.
The Fast has to run in normal mode; it can't be put into bridge mode.
The Nest might have an operation mode called Access Point. Connect the Nest WAN port to a Fast LAN port.
If the Nest doesn't have that setting, give it another local ip address (eg. 192.168.1.2) and disable DHCP. Connect Nest WAN to Fast LAN. If that doesn't work, connect LAN to LAN.
Adding to this reply, a word of caution - Google used have an Access Point mode, but they have changed this to Bridge mode now. The Google Nest Router (the one you connect with an ethernet wire to the 5G modem/gateway) will be set (in normal operation) to "NAT" mode and the Google Nest WiFi satellites will be in Bridge mode. In this setup, you will have a Mesh network on the WiFi signal from the Google boxes. The WiFi network broadcast from the Sagemcom 5G Gateway should be turned off & you shouldn't connect any other ethernet devices to the Sagemcom 5G device.
If you have to connect devices to the Sagemcom 5G device (like a printer) and if your devices (ie a phone) are using the Google Nest WiFi network, should they need to connect to the printer, then you need the Google Nest Router to be in Bridge mode (https://support.google.com/googlenest/answer/6240987?hl=en-AU&ref_topic=9832042). In this case, each Google device will need to be connected to the Sagemcom 5G Gateway's WiFi network and the Mesh functions (seamless handover) will not work.
(I use a different vendor's devices that does support mesh in AP mode & functions as described in @david64 post.)
The guidance from @david64 about connecting the Google Nest Router to the LAN port on the Sagemcom 5G Gateway is important - regardless of the mode you are using.