ok i see what you are saying, you have an adsl router/modem that you want to use with the fiber. If you can setup one of the lan ports as a wan port using pppoe then it will (should) work.
What were you using before plusnet fiber? fiber from another supplier or adsl?
If its a new fiber installation, i would probably recommend using their router until its working then try and set up your adsl using a lan port over pppoe.