Are you entering the right ip address(Default IP of Verizon fios/actiontec(actiontec is the same as verizon fios) routers is 192.168.001.250)?
If so, then did you enter the right gateway(Default gateway of Verizon fios/actiontec(actiontec is the same as verizon fios) routers is 192.168.001.001)?
^ If so, then try adding 10 to your IP in the DS's wi-fi settings screen, but absolutely don't adjust the gateway(If default ip and your verizon fios/actiontec(same as verizon fios)router is used, the IP entered to your ds will become 192.168.001.260), and see if it works then...
If all the above have been tried, but won't solve your troubles, either you need to set wep security type on router to WEP, or the firmware of router needs to be updated, or wireless interference may be overwhelming the wi-fi signal, or
maintenacewas ongoing at the time of your wi-fi use attempt, or your 3ds may need to undergo going back to nintendo for fixing, whether by damage, or individual error in the 3ds internal coding that makes it not like most of the other copies in terms of the same internals, etc.. I doubt the last one is true here, though I hope it's not wrong for your sake...
source of above