On a modern unlined fretless the side dots are where those frets are supposed to be. The reason some fretless basses aren't (especially lined ones) is that some companies churn out newbie fretless lined basses simply by taking their usual fretted entry basses and just leaving the frets off. This gives you side dots BETWEEN where the frets go which if you are used to proper side dots on the fret positions can drive you nuts.
Also some older basses before people decided where was the best place for side dots also can have dots in wrong places and usually you have to deal with that by buying a strip of LED markers or something like that.
Anyway, as others have said, you have to train yourself to use your EARS. Generally speaking on a fretted bass you finger it right behind the fret. On a fretless, typically you finger it right on top of the fret position. But there's no set rule because strings aren't perfect you'll find that the true position will vary from fret to fret...sometimes a surprising amount. This is why a couple of basses that I made fretless with new fingerboards I actually positioned the dots using a tuner and the strings I use on that bass rather than the more usual "theoretical" calculated positions. It works great but if I change type or brand of strings, it's likely it will be off again. Dots are just a guide. Your practive and memory of where each note is plus your ears are the things that determine where you play.
And Fretless is fun and not as hard to do as all the talk might lead you to believe. I've loaned my lined Carvin to bass players to "try" who have never played fretless before in their lives and they brought down the house with it!