This will show the settings required to have the FritzBox own a /29 subnet and allow a dedicated firewall device to own the other five IP addresses.

• Log into the WebGUI of the FritzBox
• Switch to advanced view.
• Internet→Account Information→Internet Connection
• Set 'Internet service provider' = Other Internet Service Provider
• Set 'Connect via' = Connection to a DSL line
• Set 'Account Information' = Yes. Username (if using Zen Internet) = zen123456@zen
• Connection Settings - Internet Connection = Maintain Permanently (recommended for flat rates). Do not tick Delay disconnection by provider to the period between
• Tick Use VLAN for Internet access
• Set VLAN-ID101
• Set DSL ATM SettingsSet manually with VPI = 1, VCI = 32 and Encapsulation = PPPoE
• Do not tick Connected network devices are also allowed to establish their own Internet connections (not recommended)
• Go to Internet→Permit Access→Port Sharing
• Add Device for Sharing
• Set 'Device' to Enter the IP address manually
• Set 'IPv4 address' to one of the Public IP addresses you want the firewall to control
• Tick 'Open this device completely for Internet sharing via IPv4 (exposed host).'
• Click 'OK'
• Repeat for each public IP.

This information is sources from this link

This information is sourced from this link.

Log onto Fritz!box UI and go to the Top Right corner and enable the Advanced View option.

Then go to INTERNET > DSL Information and set Annex to B, if based in UK and Using VDSL (FTTC) or Annex A if you are using ADSL and ADSL+ (unsure if needed, but did it just to be sure).

Then go to INTERNET > ACCOUNT INFORMATION and select

• Connect to DSL Line (check box)
• Account information (No)
• Connection Settings > Change Connection Settings
• * VLAN ID (101) for BT Fibre and Plusnet Fibre
• * Encapsulation > Routed Bridge Encapsulation and check the obtain the IP address automatically check box

Then Select PPPoE Passthrough and check the 'Connected Devices are also allowed to their own Internet connection' box.

Do not forget to disable the 2.4GHz and 5Ghz wifi and I use LAN1 for the connection from Modem (Fritz!Box) to existing Router (WAN Port)

Hope this proves a good use for the Fritz!Box 3490, as it does use the Lantiq (XWAY VRX288) xDSL chipset and works a treat on both ECI and Huawei DSLAMS in the UK