https://kaeru.world/projects/wfc

Since the Nintendo DS online server and ceased operations many years ago, you can use private servers like Wiimmfi to be able to connect to a recreation of the online services.

How to connect to Kaeru WFC, a proxy to Wiimmfi:

Open Wi-Fi settings of your DS, DSi, 3DS or N3DS. Follow the instructions manual for how to do so for each console.

Change the configuration of your Wi-Fi network, or connect it right now.

In the configuration (or after connection, go into configuration now), deselect Auto-obtain DNS and write this address in Primary DNS:

178.62.43.212

And either one of these address in Secondary DNS:

1.1.1.1 (Cloudfare DNS)
8.8.8.8 (Google DNS)

(If you can suggest a more privacy respecting DNS address to use here, please do so!)

You can now connect to Nintendo Wi-Fi connection!
Wiimmfi for Pokemon is working as a proxy to Poké Classic Network. Check out the website for information about your or other available GTS trades!