WEP used to be good 2006-2007, but my friend told me that it was hacked a few years back. He did a Google search and found a technical article that explained that it was no longer any good because there are probably kiddy programs out there to hack into it.
My router has about 4 types of encryption protocol selections.
My friend said that WEP was probably the weakest one and to choose something else instead.
The current setting for encryption that I'm using is "WPA2-PSK [AES]" with 64-bit key and a 15+ letter pass phrase that I have saved in a notepad text file on my PC so that I just have to copy / paste it back in to any new router or the existing one if I re-flash its firmware. By having a longer pass phrase, maybe it is less likely to guess because it contains letters, numbers, and symbols, ie., is stronger encryption.
Lots of times my wireless devices can't log into the wireless router, so I reboot it, but it still gives troubles. I then log into the router via wired computer connection, and change a couple of minor things like the channel or SSID (twice away then back to the original setting applying / saving each time) or something else that does not change the password, et voila, the thing starts working reliably for a while.
Often I not only change the channel used, I also change the speed up or down too. So if on N, I down shift to G or B. I'm not using the laptop to stream Hi Definition videos; it just does some email and web surfing.
It is not the signal strength either because I can take a laptop with wireless within a couple of inches away where the signal strength is huge, but it will still not connect correctly. Somehow, the router just gets stuck every once in a while--needing this wired connection thing to change the foolish settings tweaking or whatever to get it to go again.
It may be that many people with your same make & model router are having the same exact problems. Trouble would be to discover what keywords to use to search for the discussion of this?
The other recommendation that my friend had was to log into the router while the laptop, etc, devices were using the router, then click one of the "access list" setting to allow that particular machine's MAC address while disallowing everything else. This will help security by rejecting connections from hacker / cracker laptops that might be attacking your network connection / LAN file systems.
If the DS has better wireless encryption available than WEP, it might be a good plan to use that instead. The DS is a slow device, so try doing the reduced speed change (to G or B) & channel change things that I mentioned above to see if this improves things.