Science Fiction Romance, for as big of a sub-genre it is within Romance, can be extremely hard to advertise in. Below is a compiled list of places/services that could be used to advertise SFR.
Please note, this is not advocating for these businesses, nor am I receiving any compensation. This is a list complied from my own sources, and those other authors recommend in support groups.
Definitely recommend reading and researching each service before using.