Your ISRC code is essential to collecting royalties for any radio plays, use in videos or movies etc as it is the tracking code necessary to ensure the money reaches you.
There are a number of ways to get one. I recommend you join the royalty collection agency responsible for distributing the ISRC codes where you live.
Part of the ISRC code is a country coding. It uniquely identifies your "label" with a release number for the song. You don't need an ISRC code for different countries. Most collection agencies have reciprocal agreements with collection agencies in different countries. Ie if you get airplay in the UK, PRS or PPL should collect your royalties and pass them to your collection agency, who then pass it to the label and publisher... Then to the artist and songwriter. If you are the registered label and publisher and registered directly with the collection agency, then you get that money directly form the collection agency.
None of which happens without the ISRC code.
So register yourself as a self-publishing, self-releasing artist where you live. It will probably cost you up front.
Alternatively you may be able to get a label to do the release for you. I will ask PRS / PPL what is needed for me to do it for you with my ISRC code