Cellular baseband chips aren’t exactly something you can throw together in a few weeks. It’s the entire focus of some rather large companies (or divisions of said companies, for truly large groups like Samsung and Infineon). Even Intel had trouble with this, as shown by the delays in releasing new modems after they bought Infineon’s wireless division.
Making a mobile phone baseband chip needs a combination of good integrated circuit design (both digital and analogue, the latter part is what humbled the mighty Intel), decent access to semiconductor foundries and a thorough understanding of the 3GPP standards, which are hundreds of thousands of pages in total and horrifically convoluted.
Repeating what these reseller companies do does not give us a free modem. The parts which handle the actual network interactions will be supplied as a signed Qualcomm blob.
But hey - if it’s so easy, you’re quite welcome to produce the first truly open mobile phone baseband chip and network stack. You’ve got a potential customer base right here.