I had this problem once. Try enabling DMZ for the 3DS' IP. Worked for me!
and how do i do that?
I read that you don't know the password to your router's settings. Try googleing the DEFAULT PASSWORD of your router model. Usually it's admin for login (no password).
Anyway, once you're there, just search for DMZ (it's probably in the Advanced or NAT section/tab). Check "enabled" and assign it to your 3DS' IP (there should be a box there for setting the IP). If you don't know that IP, you should manually set one: on your 3DS, go to System Settings -> Internet Settings -> Connection Settings -> (select the one you're using) -> Change Settings -> Tap that arrow on the right of the touch screen -> IP Address -> Check 'No' -> Detailed Setup -> For the IP address, put XXX.XXX.XXX.150 (the Xs being the first 9 numbers of your router's gateway) -> For Subnet Mask, put 255.255.255.000 -> For gateway, put your router's gateway -> OK -> OK -> Now you must set the DNS... 004.002.002.001 for the Primary and 004.002.002.002 for Secondary (got these from Nintendo's Online Support... I guess) -> OK -> And finally... SAVE! Now just have to put the XXX.XXX.XXX.150 IP on the DMZ settings I mentioned earlier.
Phew... I hope this helps!