Openwrt behind an ISP router with no static routes

I'm trying to set up an openwrt router behind my ISP's router (Plusnet Hub One - rebranded BT HomeHub 5) and I'd like advice on the best way of setting it up. I realise this is not ideal, but it is a temporary solution as I'm constrained by electrical sockets and space near the fibre internet socket.

The openwrt device is a GL.iNet GL-MT300N-V2, a portable hotspot device. The plusnet hub is stock, I realise openwrt can be installed, but it requires some physical modification which I don't have the capabilities / time for at the moment.

The plusnet hub is quite limited in functionality, so my goal is to have openwrt handle dhcp/dns/static routes. However, the openwrt device does not have a fibre modem or external wlan antennae so I need to use the plusnet hub for those.

Most suggestions for openwrt behind a router suggest using as a DumbAP, but in my case I do want to use DHCP and DNS on openwrt. I can disable that on the plusnet hub, but there is no option to switch to a simpler modem/bridge mode. I would also like to use the openwrt device as the default gateway so that I can add some static routes to some additional subnets I want to run on virtual machines.

So, how is it best to set up openwrt? It has two ports WAN and LAN so my first idea was to:

  • Disable DHCP on plusnet hub
  • Set IP of plusnet hub to
  • Connect both LAN & WAN ports on openwrt router to the plusnet hub
  • Set WAN interface to have ip of, gateway
  • Set LAN interface to use ip of, and enable DHCP on that interface.
  • Wireless clients on the plusnet hub would receive the IP assigned by and internet would be routed via the openwrt router.

Are there any drawbacks with this setup, would double NAT come into play? I don't require much in the way of portforwarding, but presumably I'd have to set it up on both routers to work.

Did you ever get this working?

No I didn't get this working in the end, at least not without changing the plan.

I purchased a couple of Plusnet Hub routers premodified for openwrt so I could have full control over the config.

What are your specific goals and constraints? Are they identical to the OP?

I've been running a Linksys WRT1900ACSv2 for a few years now as the front facing device but here in the UK the ISP company BT are starting to cut all of the copper telephone lines and pushing the use for DECT phones. This unfortunately means I will now have to run my OpenWrt router behind my ISP's one to be able to use my landline and this introduces problems of its own.

Ideally I want to avoid double NAT which tends to happen when installing a router behind another that's on a private addresses. I've got around this though by placing the OpenWrt router into the BT router's DMZ and opening TCP/UDP 1-65535 on the BT router to the OpenWrt router.

I can confirm that the OpenWrt firewall is working because I have I have some traffic rules and port forwards that are working. Additionally I have UPnP working using a STUN server.

I originally created some scripts and Cron Jobs to detect the public IP address so that I can dynamically update my iptables rules and this worked perfectly when OpenWrt was the front facing router. I was worried that I would have difficulties being able to detect my public IP once the OpenWrt is behind my ISP's. Luckily there are several domains that bounce back the IP address it was requested from. For example using the dig or curl commands these both return my public IP address which I can then store in a file in /tmp directory and call upon within iptables for example.

dig +short
curl -4Ls

The main problem I have at the moment is getting IPv6 to work and I would like to be able to set it up identical to IPv4. I would like to setup IPv4/IPv6 using the static address protocol and not using DHCP client. On the BT Smart Hub 2 router I have set the IP address , setup a static IP address in the same subnet on the WAN interface of OpenWrt so they can communicate.

All the OpenWrt interfaces pick up the default gateway defined in OpenWrt's WAN interface, they have DHCP server's enabled on them and I'm able to connect to the internet. Question is, how do I achieve this with IPv6?

I will create a new post with additional information and will update this reply once I've posted it.