I do not use the app / wifi. I use this device manually. Yes, you can change the filter. But I would not buy it again as this product has already been replaced by an improved version with a bigger filter that lasted much longer than the 6 months you get with this model. The new version is called "Pro H". If you really want this old version, I can sell you mine without a filter.

You can use it completely offline, I never connected mine to the Xiaomi home app.