Conecting modem to belkin N+ cable router

Hello,
Can anyone out there help!?

I have just purchased in error the belkin n+ wireess router for cable, when i should have got the adsl version. Having opening the box, i am unable to return, and am now faced with the decision of purchasing the adsl version, or just using my bt voyager 220v modem with the n+ router. Will i still achieve good speeds? Will i achieve better sppeds with just the belkin N+ adsl, or is there another alternative, i.e buying a better modem to replace the voyager, and connecting that to the router.

Thanks