I can't find my language tag in Language Subtag Registry

I have a website were we will provide two different translations for two
dialects of Cape Verdean Creole. One is from Island of
Santiago<https://en.wikipedia.org/wiki/Santiago,_Cape_Verde> and
another one is from Island of São
Vicente<https://en.wikipedia.org/wiki/S%C3%A3o_Vicente,_Cape_Verde>
..

In the Language Subtag Registry there is only one subtag for Cape Verdean
Creole (kea) and one reginal code for Cape Verde (CV). So I wonder if there
is any best practice for language tags for dialects?

Can I use regional ISO 3166-2 tags? I found two that correspond good to the
two different dialects:

Sourhern region <https://en.wikipedia.org/wiki/Ilhas_de_Sotavento> ISO
3166-2 = CV-B
Northern region <https://en.wikipedia.org/wiki/Ilhas_de_Barlavento> ISO
3166-2 = CV-S

More info:
https://en.wikipedia.org/wiki/ISO_3166-2:CV