A few days ago I bought a new router the Linksys wrt400n. It works great but I cannot seem to get it to connect with my ds. I have tried changing the transmission rates and channels but nothing seems to work. I need new ideas.

You bought a wireless (draft)-n router.
DS only uses wireless-b (much slower) so the benefits of a new router are useless.
You are SOL.

Most new routers are backwards compatible. I am already aware of the Wifi signals. I am running it G on the 2.4ghz and Mixed on the 5ghz. I get ds error code 51300.

Did a google search of error code 51300, apparently it basically means you can't connect to wireless, but I think the second reply to this question was actually somewhat useful, the "best" reply was fairly obvious.

That yahoo answers does not help. The ds will not accept the router in default settings so i have to changed them. I have made it worked on my old router the wrt54g. So nobody has new routers with this problem?

You could continue to use your old router just for the DS, setting it up only to allow the DS to access. I seem to recall hearing that the DS only supports WEP. If this is the case, you could use your newer router for the computers and connect a cable from it to your old router to use it for your DS. This way you could use WPA/WPA2 on the new router without having to use WEP on it for the DS, using the old router with WEP and using MAC filtering to only allow the DS to connect, if you want to keep people out of your network.

Yeah, you could connect the old router into the new one directly, and use it for wireless

The second router attached to new router does not connect to the internet. I cannot figure out why. I want to do this without the old router though.