Hey, information you should know, as they are likely reasons you may not get a reply
1. Online trades need the Nintendo Online paid subscription
2. It is said that hacked Switches are at risk of being banned from online usage. People are commonly either already banned, or not wanting to risk it.
So you not getting a reply could be a combination of factors, such as people not having a paid subscription, and/or they don't wanna risk getting banned,
or they're already banned.
Alternatively, you can try to learn from another user on our forum (this thread), and try to get someone nearby/locally with a modded Switch to transfer you a hacked save you made.
One way save transfer system: https://en-americas-support.nintendo.com/app/answers/detail/a_id/45047/~/how-to-transfer-your-save-data-to-a-nearby-nintendo-switch
Also, to slightly correct your misunderstanding/the way you phrased it:
You make it sound as though the serial codes are region locked. They're not. The trouble is getting them, I acknowledge that. But if you can get it, you can use it.
For example, here are some new codes for the US Shiny Pikachu/Eevee event.
(if anyone uses them, please specify which was used)