I didn't install nothing so far, it's a new iPad, out of the box.

Hi, I am trying to install the netfree filter on my iPad Air (4th generation) IOS 15.0. So I tried to install the certificate for a iPhone, but I get a error message "profile error, invalid profile". The support team told me they don't have knowledge on this devices. Can someone here help me out with simple steps, on how to install the filter on my device?