Hello,
Services > Web Filter allows you to block OUTGOING traffic to specific websites.
What you probably want is to filter INCOMING traffic so a specific host can connect to your router and others can't.
Enable remote access in System > Administration > Access Control
To restrict access to specific IP's go to Network > Firewall > Traffic Rules
If you want to restrict WebUI access:
1. Click edit on Enable_HTTP_WAN
2. Enter IP address that you want to allow in Source Address.
Same thing if you want to configure SSH access, just edit Enable_SSH_WAN