hello Geloma
Welcome to the Community.
Modem Mode is not available on the FIBRE box. First of all you need to check if IPv4 is activated on your connection. Log on to your Connect Box and go to Admin > Info , have a look at WAN IP settings. If it says only IPv4 you’re good to continue.
Next thing to do. Disconnect your Unifi express from the Connect Box. Log on to the Unifi express. Change the LAN settings on it. The subnet 192.168.1.0/24 can’t be used, so please change the settings to something like 192.168.10.1, subnet mask is still 255.255.255.0 and the DCHP can be set to 192.168.10.6 to 192.168.10.254. Now change the WAN-settings on the Unifi express. Make the WAN connection static. IP 192.168.1.2, subnet 255.255.255.252, gateway 192.168.1.1, edit the DNS servers if you want to 8.8.8.8 and 8.8.4.4 are the google DNS. Connect the WAN port of the Unifi express to port 4 on the Connect Box 3 and reboot the Unifi express. The internet-connection should now work.
Sign in to the Connect Box 3 (192.168.1.1) and go to Advanced settings > Security > DMZ enable DMZ and set it to be 192.168.1.2 and apply the settings
Next thing disable the Wifi on the Connect Box 3. Go to Advanced settings > Wireless > Wireless signal disable both bands and apply the settings.
You could also disable DHCP on the Connect Box. Any device connected to the Connect Box 3 will not get an IP address automaticallly and there for will be unable to gain internet access. The Lan settings need to be applied manually for a device connected to the Connect Box directly, like you did on the Unifi express WAN.
Once the settings are applied, reboot the Connect Box 3. Your Unifi express will now serve as the main gateway and all traffic will be handled by it.