Ok to make this short, type in ip in google and copy your ip adress to the search bar and press enter, typle in admin as username and admin as password, then hit enter.
Then go to advanced settings and click ok, then click advanced, go to firewall and dmz and enable dmz, put in your 3ds's firewall (which will be in lan settings under dchp, it is in the one left advanced) then click save settings, then go to port fowarding, put in your 3ds's ip adress and then put as the first box 1 and the second one 66535 and then click save settings and it should work!
Then go to advanced settings and click ok, then click advanced, go to firewall and dmz and enable dmz, put in your 3ds's firewall (which will be in lan settings under dchp, it is in the one left advanced) then click save settings, then go to port fowarding, put in your 3ds's ip adress and then put as the first box 1 and the second one 66535 and then click save settings and it should work!