Edit: Access to mobile networks is a deep rabbit hole and you might immeditately run into issues regarding patented algorithms. But you can integrate an off-the-shelf module and keep it at arms length to your system. If you're fine being stuck on 3G. The bigger issues are you will have similar issues with the hardware. Unless RISC-V takes off and yields some affordable, auditable hardware, the ROM concerns still apply.
I'm sure you're breaking at least some of patents or use some intellectual property of the US corporations and their lawyers will make short work of you.