Use ISP's router as a switch for a OpenWrt router

[psherman]

@slickrick

I can easily change the IP of my ISP's router to the same subnet as the OpenWrt.

You can do this, but then the OpenWrt router cannot perform as a router.

@slickrick

The reason I was going for two subnets was that my devices should always route through the OpenWrt.

That's fine.

@slickrick

In summary I would have a double NAT. Because of firewall settings I need my OpenWrt to do NAT.

Double NAT is not optimal, but can work.

@slickrick

lan->wan,lan2
lan2->vpn,lan

ISP WAN > ISP LAN > OpenWrt WAN > OpenWrt LAN makes sense (your first line)
OpenWrt LAN > VPN makes sense, but it does not make any sense (nor will it work properly) if you try to tie it back to the ISP LAN.

@slickrick

Without the NAT on my OpenWrt a device on lan2 could access the internet without going through the VPN. Therefor I was choosing two subnets with one only known to my OpenWrt and the ISP's router.

@slickrick

Without the NAT on my OpenWrt a device on lan2 could access the internet without going through the VPN.

If you are trying to ensure that certain devices/traffic always goes through the VPN, yes, it makes sense why you have setup the OpenWrt router.

@slickrick

Therefor I was choosing two subnets with one only known to my OpenWrt and the ISP's router.

I don't really know what this means. But it doesn't matter.

None of this is relevant when referring back to the original problem/question -- you wanted to know how to configure OpenWrt to allow you to use the ISP router as a switch. There is nothing you can or should change on OpenWrt to achieve this. It is really a question about configuring your ISP router to handle VLANs (which many don't do, or at least not with user-accessible VLAN configuration options). And as I have explained already, it is very bad practice to tie the networks together over an unmanaged switch...but it probably can work, just ill-advised.

[Hegabo]

@slickrick

I just need the right OpenWrt configuration to be able to have the WAN interface on e.g. LAN1 of the OpenWrt and also have LAN1 working as a regular LAN port.

That was the config for the Raspberry Pi device in the topic I shared earlier.