Can I use my member card with social security providers?

No, your Sanitas card is for exclusive use with Sanitas providers only. You can only use public-system doctors and hospitals if you have a right to public healthcare - for example because you pay social security in Spain, you are a UK or EU state pensioner with an S1 form, or you pay into the convenio especial scheme. If none of those apply to you, private health insurance is how you get cover.