How to force my AT&T iPhone 6 Plus to roam domestically?

I have really poor coverage in my area, so i wanted to know if there was anyway to force roaming on my phone without buying another SIM card or anything like that.

What do you mean roaming? I don't get you. If AT&T has bad coverage in your area, just terminate your contract with them and get the carrier that has better coverage in your area. You can use your iPhone 6+ by unlocking it. For more detailed info, feel free to mail me.

Roaming only applies when you travel outside the country. In the US you can only use the AT&T network.