PPTP client not working when router is connected directly to ISP, but when through other router

Hello ladies and gentlemen,

I have a problem that really confuses me and despite lots of reading, researching and some trial and error, I haven't found any solution. My router is EdgeRouter X with EdgeOS 1.9.1.

The thing is that I am configuring my router with a PPTP client (pptpc0) to connect to a remote computer over Internet, however:

- When I connect the EdgeRouter X through a Netgear router at home (i.e. LAN --> EdgeRouter --> Netgear --> WAN/ISP) everything works perfectly fine and the PPTP client connects beautifully.

- When I connect the EdgeRouter directly to Internet (i.e. LAN --> EdgeRouter --> ISP), everything else works perfectly except the PPTP client, which doesn't connect at all.The Dashboard say that the IP address is TBD and no dynamic routes show either.I have tried connecting to different internet providers, one with static IP (office) and the other with DHCP (home), with exactly the same result. Settings are exactly the same as above (except the obvious change of DHCP/static for current ISP). There is no MAC authentication going on at any of the ISP's. Both ISP's assign public IP v4 addresses.

What have I missed? As I understand it, the router shouldn't care whether it has a cheap Netgear router or an ISP's router upstream?

I'm adding most of the config file below. Actual servers and passwords have been changed.

firewall {
    all-ping enable
    broadcast-ping disable
    ipv6-receive-redirects disable
    ipv6-src-route disable
    ip-src-route disable
    log-martians enable
    name WAN_IN {
        default-action drop
        description "WAN to internal"
        rule 10 {
            action accept
            description "Allow established/related"
            state {
                established enable
                related enable
            }
        }
        rule 20 {
            action drop
            description "Drop invalid state"
            state {
                invalid enable
            }
        }
    }
    name WAN_LOCAL {
        default-action drop
        description "WAN to router"
        rule 10 {
            action accept
            description "Allow established/related"
            state {
                established enable
                related enable
            }
        }
        rule 20 {
            action drop
            description "Drop invalid state"
            state {
                invalid enable
            }
        }
    }
    receive-redirects disable
    send-redirects enable
    source-validation disable
    syn-cookies enable
}
interfaces {
    ethernet eth0 {
        address dhcp
        description Internet
        duplex auto
        firewall {
            in {
                name WAN_IN
            }
            local {
                name WAN_LOCAL
            }
        }
        speed auto
    }
    ethernet eth1 {
        description Local
        duplex auto
        speed auto
    }
    ethernet eth2 {
        description Local
        duplex auto
        speed auto
    }
    ethernet eth3 {
        description Local
        duplex auto
        speed auto
    }
    ethernet eth4 {
        description Local
        duplex auto
        speed auto
    }
    loopback lo {
    }
    pptp-client pptpc0 {
        default-route none
        description "External VPN dialup"
        mtu 1500
        name-server auto
        password myPassword
        require-mppe
        server-ip vpn.server.tld
        user-id myUserid
    }
    switch switch0 {
        address 192.168.1.1/24
        description Local
        mtu 1500
        switch-port {
            interface eth1 {
            }
            interface eth2 {
            }
            interface eth3 {
            }
            interface eth4 {
            }
            vlan-aware disable
        }
    }
}
protocols {
    static {
        route 172.1.2.3/32 {
            next-hop 172.173.174.175 {
                description "The remote computer to connect to through VPN"
                distance 1
            }
        }
    }
}
service {
    dhcp-server {
        disabled false
        hostfile-update disable
        shared-network-name LAN {
            authoritative enable
            subnet 192.168.1.0/24 {
                default-router 192.168.1.1
                dns-server 192.168.1.1
                lease 86400
                start 192.168.1.38 {
                    stop 192.168.1.243
                }
            }
        }
        use-dnsmasq disable
    }
    dns {
        forwarding {
            cache-size 150
            listen-on switch0
        }
    }
    nat {
        rule 5010 {
            description "masquerade for WAN"
            outbound-interface eth0
            type masquerade
        }
        rule 5011 {
            description "masquerade External VPN"
            log disable
            outbound-interface pptpc0
            type masquerade
        }
    }
}

Please help! Any ideas and pointers are welcome!

I wish you all a happy new year!

Tom