Long term, only paid ads and SEO will work (and SEO can be fickle)
Short term, run some paid experiments (knowing you will probably not get positive return yet) and maybe some influencer marketing (they'll cost money, but not as much as paid ads depending on the niche)
So, usually paid experiments won't really give you a return at an early stage, but occasionally they can if you get lucky, but at least they'll give you an idea of what your CAC can be, and give you a starting point to start optimising it