Hello Paula, welcome to the forum.
In your current configuration, the FritzBox is the default gateway for the LAN network segment;
any communication from LAN to guest network would be routed via the FritzBox.
For this reason, it also decides the firewall policy between these networks. The OpenWrt firewall currently has no effect since LAN->guest traffic does not pass through the OpenWrt device.
As far as I know (but I have not tried it), the guest network of the FritzBox is isolated from LAN, and there is no way to change this behaviour. Instead, the routes on the devices connected to your network must be reconfigured such that LAN->guest traffic passes through the OpenWrt device. I could imagine multiple ways to achieve this:
- Configure a static route to the 192.168.179.0/24 network via 192.168.178.2 on each LAN client device (not very practical).
- Distribute the same static route to clients with DHCP option 121: Classless Static Route. I have not tried this and I am not sure which clients support it.
- Use the OpenWrt device as the default gateway for the LAN segment. This needs further considerations on how OpenWrt's own uplink (WAN) is connected. We can discuss this in more detail if you want to try this option.
In any case, you need to make sure there is only one DHCP server on each network segment, which could be provided either by the FritzBox or by OpenWrt. Check the guides in the OpenWrt wiki on how to set up a guest network; you would use that configuration if you want OpenWrt to be the default gateway for the guest network segment.
In case you get confused about which path the network traffic might take, I suggest to make sure there is only one such path. In particular, you could (temporarily) disconnect the guest network from FritzBox LAN4 and disable the guest WLAN in the FritzBox while you are trying to set up a guest network on OpenWrt. While this may break connectivity for wireless clients connected to the Fritzbox guest network, it makes network debugging easier.
Where are the devices located (FritzBox, Switch, Archer C6): closely together, or in separate places?
Do you want to offer the WLAN from multiple devices (FritzBox, Archer C6) for better coverage?
Does your cable provider offer IPv6 service, and a delegated prefix (DHCPv6-PD)?
Do you get a public IPv4 address, or just a private one, perhaps delivered via Dual-Stack Lite (DS-Lite)?
Cable ISPs in Germany usually provide better service via IPv6 than IPv4. If that is also the case for you, then I suggest making sure that IPv6 works well in your local networks, in addition to IPv4.
Which internet speed did you buy? I wonder if the Archer C6 is powerful enough to route at that speed.