That would depend on the sophistication of your ADSL2+ Router. And by sophistication I am not trying to say that your new router isn't up to the task, I'm saying that it depends on whether the software on your router will allow you to choose a different interface for the internet connection.
For example, many router/modem combination devices expect their internet connection to be established via the integrated modem, many are hard coded specifically for this purpose. This means that some router/modem devices cannot be configured to accept an Internet/WAN interface that isn't sourced from the integrated modem.
However there are some routers that will allow you to change the Internet/WAN interface to one of the network ports on the router. And for those routers that don't have this functionality, there may still be an option to obtain a custom firmware for the device that may permit this functionality.
First things first though, plug your new router in connect to it, and have a look at its configuration. what you are looking for is the WAN/Internet configuration, and you are looking for an option that allows you to select one of the ethernet ports on the router as the WAN/Internet Interface. If you can select an ethernet port as the WAN interface simply pick one, make a note of it, then connect the network cable from your modem to the same numbered port on your router. If it isn't available then you may need to look for a custom firmware that will likely result in a voided warranty if you use it, assuming you can find one.
Incidentally, why not just use the modem/router device your ISP supplies? Most modems are integrated into routers anyway and if that is the case here you aren't necessarily improving connectivity performance by using a separate device. I'd also suggest perhaps using the ADSL2 router as a Wi-Fi range extender if possible. Oh and, could you post the manufacturer and model of the ADSL2+ router and whatever it is the ISP will supply you, it will help if you would like some further ideas or troubleshooting! Good luck!
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