Developers

I think it just happens organically

You can't really influence it as a government with some program where you spend $100K on building a free coworking and inviting a few foreigners and calling something "Nomad City" or "Nomad Island" (I've seen this a lot)

It doesn't work, well it works for a few months, then everybody leaves again

Only way you can attract foreigners is the same how you just generally create a great place:

Make it safe, affordable, easy to do business, easy to do gov stuff etc. add easy visas, residence, taxes for them

Also gotta make sure your locals are actually happy, or they'll just get angry seeing DNs get benefits and then protest/attack them