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I have an existing DSL package that provides a Public IP. I use this to perform port forwarding and run VPN to access my home network and devices over the internet. However, the speed is only 4mbps, and the ISP will not upgrade any further due to low demand in my area.

I then subscribed a broadband service that give speeds of up to 40mbps. However, the ISP provides a Private IP that sits behind a NAT, and so I cannot access my home network and devices over it. And they are not providing any work around for this, i.e. static IP, etc.

My question: Is there a way to somehow connect the two DSL and Broadband modems together..? Such that all devices in the network are still routed through the Broadband modem for internet access, obviously for it's much higher speeds; and i am still able to connect to the ADSL modem via its Public IP and access the rest of the home network and devices via VPN or port forwarding.

Is it just as simple as connecting the two modem via ethernet cable, assign separate IPs, run DHCP on only either one..? Any issues connecting two modems having its own WAN connection..? Will the devices connected to the network automatically routed to use the faster connection..? Any specific settings i need to perform to make this work..?

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    Simple answer... No. Long answer... No. Sorry, I can't see any way this could possible do what you looking for.
    – acejavelin
    Jan 8, 2018 at 3:14

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I am assuming that when you say modem, you are referring to a combination modem + router unit. Let's call the router with the faster connection A, and the slower one B.

For starters, you will want to connect two routers together via Ethernet, connecting to the LAN ports on each. You will need to disable DHCP on one of the routers. You will likely want to leave it enabled on A, so that it can be used as the default gateway. The internal IP address of B must be in the same subnet as A's network, and should be set statically.

If the VPN is running on B itself and performs source NAT when talking to the internal network, VPN clients should already be able to reach your network properly.

However, port forwarding is a bit trickier, as there is a routing problem. Even when packets are forwarded to an internal host from B, the internal host has a default gateway of A, and the replies will be sent through A instead of back through B, breaking the connection. This can be solved with source NAT, which would replace the source address of forwarded connections with that of B. B keeps track of each connection, so the replies get back where they came from. You will likely need iptables or another fully configurable firewall to do this.

I will clean this answer up when I get back on a computer.

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  • Thanks. I'll give the above a shot. Base on what i currently intend to use this for, getting VPN connection will be sufficient. Wont be needing any port forwarding. But what do you think of dual WAN modems, i.e. ASUS AC52U..? Some of these modems have dual WAN with load balancing mode. So i can connect both DSL and broadband at once using the same modem. If i set the DSL line as default, i suppose it will use the DSL IP for DDNS. But i still wonder if i can route all other connection requirements to the broadband line (the faster connection)..
    – Eldi MS
    Jan 9, 2018 at 4:31

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