Some things that come to my mind:
- Dirty filter on the dehumidifier
- Maybe needs cooling gas refilled on the dehumidifier if it's older than 2 years
- Exhaust fan or carbon filter dirty, resulting in low ventilation
- Open all the intake flaps on your tent
- Leakage in your walls, floors or windows which lets in the outdoor humidity. Although this is impossible to fully fix, no room is 100% sealed, but still check for bigger openings that you could maybe seal temporarily.
And hopefully you will be able to afford a small AC unit, maybe a used one in good condition and clean. Look into portable units, i assume they are cheaper but i never owned one so hopefully someone can help you with that.
You will need cooling soon when the summer comes, especially at the end of the flower and drying. And AC units will also help dehumidify some of the air while cooling. Also that dehumidifier surely puts out some heat, at least mine does.