Accessing modem from inside firewall
Some DSL or cable modems have web interfaces on private IP addresses. Since these sit outside the firewall and don't have a public IP, accessing them isn't as straight forward as it might seem. The firewall is typically assigned a public IP, and sends all outbound traffic upstream to the ISP. The ISP won't route the private subnet back to the modem, leaving it unreachable. This page describes the work around needed to access the management interface on the modem from the inside of the network.
Note: The modem's management IP must be on a different IP subnet than the internal network. If it is not, attempts to connect to it will never go to the firewall to be routed out to the modem, as hosts on the internal network would try to connect to it on the local network and fail.
Configure a new Interface
A PPPoE WAN is actually assigned to a virtual PPPoE adapter, not the physical port.
Under Interfaces > (assign), create a new OPT interface, and assign it to the physical network card that is on WAN. For example, if the WAN on the assignment page is "PPPOE0(re2)", choose re2, and Save the changes.
Go to Interfaces > (new OPT interface), and Enable the interface. Give it an IP address in the same subnet as the modem, such as 192.168.1.5/24. Do not set a gateway. Rename the interface to ModemAccess or a similar useful name.
Now NAT needs to be configured to translate traffic destined to the modem to the new interface. This is necessary so the modem sees the traffic sourced from an IP on its local subnet. Without this NAT, it would be necessary to configure a route on the modem so it knows how to reach the internal subnet. With some modems this isn't possible, and in most cases it's easier to NAT the traffic so routing isn't a concern. To add the NAT, browse to Firewall > NAT, and click the Outbound tab. Switch to Manual Outbound NAT and click Save. A rule for LAN to WAN is automatically added.
Click to add a new Outbound NAT rule. For Interface, specify ModemAccess. For Source, specify Network, with the LAN subnet entered. The Destination is the IP subnet of the modem. In the Translation box, select Interface Address.
Then click Save and Apply changes.
It should now be possible to access the modem from LAN.