Technical Tip: Modem private IPs cannot be accessed through the PPPoE interface, possible workarounds

FortiGate, WAN with PPPoE.

Tested on v6.4.8 build1914 | v6.4.10 build2000.

Adapt the configuration to limit the source in the rule so the internal clients will be able to reach that private network.

1) Create a Static Route for the network:

Go to Network -> Static Routes and select 'Create New'.

2) Create an IP pool:

Go to Policy & Objects -> Virtual IPs, select 'Create New' -> 'IP Pools'.

3) Create a Firewall Policy:

Alternatively, create the policies in the CLI with the following commands:

# config router static
    edit 1
        set dst 192.168.1.0 255.255.255.0
        set device "wan1"
    next
end

# config firewall ippool
    edit "IP_Pool_192.168.1.13"
        set startip 192.168.1.13
        set endip 192.168.1.13
    next
end

# config firewall policy
    edit 1
        set name "To Router"
        set srcintf "internal"
        set dstintf "wan1"
        set srcaddr "all"
        set dstaddr "NET_192.168.1.0/24"
        set action accept
        set schedule "always"
        set service "ALL"
        set logtraffic all
        set ippool enable
        set poolname "IP_Pool_192.168.1.13"
        set nat enable
    next
end

Result:

internal in 10.16.4.56 -> 192.168.1.5: icmp: echo request
wan1 out 192.168.1.13 -> 192.168.1.5: icmp: echo request
eth0 out 192.168.1.13 -> 192.168.1.5: icmp: echo request
wan1 in 192.168.1.5 -> 192.168.1.13: icmp: echo reply