It is very rare for the PlexiDor pet door to freeze shut, but it can happen in extreme cases when there is a combination of cold and condensation.
Make sure the installation hole has sufficient insulation. You may want to add foam weather seal on the back of the pet door frame, to get a barrier between the frame and the door or wall where it’s installed.
Put the tape across the bottom of the dog door frame. If the door still freezes at the top, put heat tape all the way up the sides and across the top as well. If possible, it’s best to put the tape on the backside of the frame – that way your cat or dog won’t be tempted to play with it.
Cold in itself is unlikely to freeze the door shut – the problem comes from the combination of low temperatures and humidity.
If the door is located close to a dryer vent, you may need to wipe it off after running the dryer. If there is high humidity indoors, a dehumidifier can help reduce condensation.