If the web filter seems to have issues reliably accessing the internet after you have added new connections, network cards or routes to the Smoothwall Filter and Firewall, you might just need to wait for the web filter to pick up on the changes you've made and you might want to try a proxy server restart.
When the network configuration changes, the web filter picks up on those changes but it may take a minute or two before the soft restart of the web filter has completed.
When any changes are made to the network setup or routing, always perform a proxy service restart to make sure changes are reflected correctly in the proxy server configuration.