Unfortunately, it is common for insurance companies to ignore the treatment recommendations made by the doctors they choose. There is a procedure called "alternative medical care" which can be used to seek an order requiring the insurance company to pay for the recommended treatment. The first thing you are required to do is to send a letter to the insurance adjustor requesting that the care be approved. You should keep a copy of the letter as you have to prove you requested the care in writing.
This and other specific legal requirements regarding alternative medical care are explained in Chapter 4 of our Iowa work comp. book available at no cost to you.