Split ports into different functions


I have a rooted Thomson/Technicolor DJA0231 used as a VDSL modem router with 2 of the 4 LAN ports occupied.

I would like to play with PfSense so I was wondering how I could continue to forward WAN from the modem to the PfSense box on port 1 and then have PfSense Lan side return to the modem's 3 other LAN ports for distribution?

Is it simply a case of removing port 1 from br-lan and then stopping dhcp and DNS on ports 2-4? Luci on this modem is not stable so I will have to use cli.

I know I could simply achieve this using a switch or another ethernet card and putting the modem into bridge mode but space is tight and it seems there is already the hardware there.

Thanks in advance.

Is this modem/router really running a vanilla OpenWrt installation? If not, the following might or might not work:
Just bridge your dsl0 interface (or the corresponding VLAN, depending on your ISP) to one of the LAN ports (this requires splitting off this LAN port into a different VLAN) and use pfsense to establish the connection (PPPoE or DHCP or whatever is requried). Then feed back the LAN port of pfsense to another LAN port of the VDSL modem. You might have to disable/reconfigure dnsmasq on the modem/router, depending on your setup.
If you want to use just one port, you can use a VLAN trunk between your Thomson and the Firewall (bridging rules still apply).

Edit: Check out the Wiki on how to configure VLANs: https://openwrt.org/docs/guide-user/network/vlan/switch_configuration

It's an ISP's customised version but I believe most of the functionality is still intact.

I'll give that a go and report back.

Thanks very much for the suggestion.

That's a quite bold statement, for a device that isn't even supported by OpenWrt at all - nor ever will be (Broadcom, SOC, xDSL, DECT, FXS, wireless, …). Please talk to Thomson/ Technicolor, we can't help you with that.