The magic solution everyone learning fretless bass needs: Muscle memory. It's both magic and totally completely real. Otherwise nobody would be able to sing in tune. Just as a singer trains his or her vocal muscles, you simply have to train your hands and ears to do it, and get your brain out of the way.
Practice blindfolded, or at least with your eyes closed, and practice shifting positions. This will develop your muscle memory and your ears. Listening is critical. Learning to shift positions is great for fretted too- and you may find it's actually easier on fretless- because if you miss a shift on fretted, it's totally obvious, and every note after it is off until you shift again. On fretless by contrast, once you learn to adjust on the fly by ear, you can be 'close enough', and by rolling your finger or minor adjustment, you can hit the intended notes. Yep the notes are closer together up the neck, but it also means the adjustment you need to make if you're off a bit is smaller and easier to achieve- if you practice it.
Find music that requires a lot of shifting to practice with- you'll be surprised how quickly you just 'magically' find the right notes, or close enough more often then not. Steely Dan's Rikki Don't Lose that Number is a good one, it's all over the neck, with a lot of I-V interval jumping - which even within the same position can be a challenge on fretless. Josie is another good one, because it has double stops up the neck- getting those in tune really pushes your ear/hand coordination- but it's so rewarding once you get it.
Relying on visual indicators may actually hold you back from developing this skill, because you might never gain the confidence in your hands and arms being able to work directly with your ears without having to think about it. If you intend to play fretless live, make sure you can year yourself well. I rely on the side dots a bit live if I'm having issues hearing myself well, but it's actually way easier and way more fun to just trust the muscle memory and my ears.
I'll admit I started on violin as a wee lad, then switched to cello when I was big enough to play one, then added double bass in high school. You might think that gives me a leg up on fretless electric, but I still need to work at it, especially since I have basses with scale lengths from 25" to 42". But the good news is that it's totally doable if you practice it - and learn to trust it (it being muscle memory). Each time I switch instruments, it takes a minute to adjust, but I know that I can do it, and that confidence alone allows me to get my conscious brain out of the way and just play.
Someone earlier in the thread said they just wing it and hope. It's actually not a bad strategy! It's really the only way violinists, cellists, double bassists play fretless instruments with no lines or even side dots.