I'm not sure where exactly you have advertised, but here are a few suggestions.
PlanetSuzy has a lot of 3D porn threads.
HentaiFoundry - maybe make an account and post one of your free stories and leave links to your Patreon
Literotica actually has these picture/story hybrids that you can post, so you can post a free story there
The /aco/ (adult comics/cartoons) board on 4chan also has 3d porn threads that you can advertise in
NaughtyMachinima is all about 3d porn and they have a picture section
You can also make an image gallery on ImageFap and put the story in the picture description.
Just a few ideas.