If you have a static IP where you host your jellyfin service you shouldn’t need your dydns anymore.

a domain provider doesn’t know what you are doing. It knows you want to access jellyfin.your.domain but has no clue what you are watching or the specific URLs you are going to.
Think of it like a library reference card, the library knows you want Encyclopaedia Britannica volume 12, but they don’t know what you are actually looking up.

I have a domain with porkbun and dont have issues. When my reverse proxy needs a new certificate I do nothing because Traefik uses the porkbun API to do the Let’s Encrypt DNS validation.