Dehumidifier help

Hi. I'm a complete newb. I have a 6x6 room I've made into a grow room. Has a 200mm AC infinity extractor and two passive inlets. I have yet to get oscillating fans and maybe this will help? My RH is sitting about 70% and temp 19°. I am thinking I'll need to get a dehumidifier but my room is pretty much an outside space.... So I can't just put a dehumidifier outside it as this would then be dehumidifing the outdoors!! And I'm not sure having one inside would work given the inline fan size etc....(or would it?)

Would putting one in a cardboard box beside the room work!? Or would it still just be like I'm trying to dry out the world!! 🤣

It's cold/damp where I live so I'm anticipating more humidity as winter comes and I struggle to keep the room warm enough to keep the VPD adequate.

Thanks