Hi, today I planned to post a topic about this. 'cuz I actually solved my problem this morning
it's exactly the same problem with yours.
My old 3ds is on the latest firmware btw ! And im not using any external usb wi-fi modem, i'm using my own wi-fi modem and it connects to the internet too ! Heres the steps that I take this morning to make it work ! My pc is Windows 10 btw !
1- On 3ds internet settings, write your modem IP (the IP you write on your browser to reach it's settings) to the DNS, primary and secondary ! And connect to your modem via wi-fi !
2- Open cmd as admin !
2a- Eject the Ethernet Cable and open your wi-fi to connect the modem !
3- Write nbahomepass BASE256.txt true 600 30 (600 or 900 or between for reaching it in time) and start it ! Press a button if you want to cycle mac adresses if you want but i advice you to wait !
3a- Set up Nzone if you didn't 'cuz you have to see the ConsoleNintendo3ds (or smth like that) in the connections after you started it.
4- If you want Eon Ticket on ORAS, delete others and leave ORAS only, otherwise leave everything !
4a- If you want Eon Ticket, make your last opened program anything than ORAS, just open up system settings and close it !
5- Heres the nice part ! Press the power button but don't press shutdown on screen, just close the lid and put 3ds near the modem (sleep mode) ! you'll see a nonstop green light (no flashes) after a minute or two!
6- Open your 3ds after waiting 2 to 3 minutes when green light is on, when you open the lid, you'll see green light is gone and here's your spot passes contacts !
7- Repeat this steps (5-6) on every mac cycle changes to get more spotpass/streetpass contacts!
7a- Repeat this steps (4-4a-5-6) untill you get Eon ticket from someone ! Check the Buzz for it !
Have fun and I hope it helps people, 'cuz I got my Eon ticket this morning
Note: In the latest updates, nintendo probably changed how it works, (Home menu for sending, sleep mode for receiving) 'cuz in v9 firmware, i be able to get streetpasses even in game and in homescreen, but not anymore !