Car antenna? Please Help! No Signal past Iowa state line. Highway 65 to US-34 E to Ottumwa, IA

I drive from Northern Missouri to Ottumwa, IA up highway 65 up past Trenton, MO to highway US-34 E. I have zero signal once I am at the Iowa state line all the way into Iowa. I am wanting to know if there is a car antenna or anything at all to buy to give me some bars or signal to use my phone in my car or out of my car since I travel so much. I have iphone 6+