I have tenda w150D wireless router+modem. Its antenna is broken & it doesn't support dd-wrt. Now I wish to buy a router that does support dd-wrt. So can I use this current router+modem as modem only and buy a new dd-wrt supported router?
If you connect a cable router to the LAN port of this one, you should be able to set it up as it's own network. You'll want to set up DMZ on the first router to point all traffic to the second one, and disable it's WiFi if you won't use it.
As an example, I have a router in my room connected to the main router downstairs, and it allows me a fast internet connection without having to use WiFi.
The problem you may have is if the networks are configured to use the same subnets you might find that you can't access the settings / LAN on the second router. Have a look here if you're not sure what this means.