The word “fasting” isn’t in the Greek text. It is added by some translators.
These passages aren’t about fasting. They contain the Lord’s explanation of why his disciples could not expel a spirit.
The point is: anyone involved in spiritual warfare may need to know that a deaf spirit can only be expelled by prayer.
So you don’t think fasting and prayer are potent spiritual weapons?
Have you ever experienced anyone who had a deaf and dumb spirit?