Depends on the bandwidth you have and your actual requirements. You may need specific 3GPP interfaces, which will be available in commercial products, and you can straight away start with your application. Vendors generally take care of IOT issues. (Well, interoperability is an issue in Diameter implementations...)
On the other hand if you have the time and resources, you can build things on top of OpenDiameter.
Disclaimer: We are a Diameter vendor but I believe this comment is fairly objective :)