Is there a manual or exemples to restrict WebGUI Access to host(s) IP or by mac adress, only inside LAN?
I found some but for pfsense. Propably it´s the same...
There are a few ways I can think of.
1) Configure a FW rule blocking all traffic to your firewall management IP&port. If wanting to configure by MAC, you could set your DHCP server to assign a specific IP to your management computer (Services: DHCPv4 > DHCP Static Mappings for this interface).
2) Create another VLAN, create FW rules only allowing your management computer IP&port to access the new VLAN.