Converting Thompson TG585v7 to a switch

[#1]

Hello
I have a Thompson TG585v7 that was supplied to me by Plusnet. It has Plusnet firmware on it, the latest and greatest. I recently converted to fibre broad band from Plusnet and have gotten another wireless router. I now want to convert the TG585v7 into a switch to connect my devices (3 ports arent enough on the new router...) So far I have disabled DHCP and shut off the WLAN, but the router keeps complaining about my broadband connection being down (obviously!) . How do I configure it so that it understands that a route to the outside world exists via one of its Ethernet ports?  I know this box is a 4 port switch so it should be some config setting somewhere...
Do I need to flash the firmware to the stock one?

[#2]

This will do what you ask, connecting the TG585v7 to another router via ethernet ports. No need to change firmware.
http://npr.me.uk/together.html
That's not suitable for connecting the TG575v7 directly to the fibre modem though.

[#3]

As you've done, turn off the DHCP server.  If you don't want to use the unit as an extra wireless access point then turn off the wifi and give it an IP address in a completely subnet from the proper network (as you won't need to get to it again) and then plug a cable between LAN ports on both the old an new routers.  Alternatively, give the old router an IP address outside the DHCP scope of the new router (i.e 192.168.xxx.250) so you can still get at the GUI to turn the wifi back on should you want to add a second WAP to get better coverage.
The important thing is don't plug anything into the WAN port of the old router...

[#4]

Hi there,
This isn;t something we could officially support you on ourselves but please do post back if you're able to get this sorted as it could come in helpful to other customers.