It's important to realize that Jesus was talking to his disciples. Anything they did that was miraculous was to further the kingdom of God. Jesus gave them the power to perform miracles. Christians today don't have that.

Paul and Timothy had health issues but it would have been sinful for them to heal themselves or each other. Paul had bad eyesight. Timothy had frequent illnesses.

Jesus never performed self-serving miracles. That's why he refused to turn stones into bread.

It's being presumptuous to think that you can cure chronic illnesses with sufficient "faith". If you could actually cure people with illnesses that would be something you'd do for unbelievers only.

For Christians today, Jesus was talking about faith that God will provide whatever you need to do the work that he wants you to do. You can't move a mountain if God doesn't want the mountain moved. You'll have the chronic illness if it's part of God's will that you have it.