It looks like your Calathea is still in its nursery pot (that's a good thing!), so if it is going two weeks without water you are letting it get too dry. That is what is causing the brown leaf edges. Those can be trimmed off as they will not recover. Soil, pot size, and humidity (misting) are not a problem, but watering is.
Keep your plant as close to the window as possible but just beyond the reach of the direct rays of the sun.
Water it thoroughly as soon as the surface of the soil feels dry or starts to turn a lighter shade of brown. Let some water drain through the holes of the pot, but make sure it doesn't sit in water. If you are using a moisture meter, it is not reliable.
