First you need to get a job in some telco company and work on related projects. That knowledge is not available on github or books. Actually it is the same with other high-pay integration areas like finances, banking, logistics etc. Once you gain some expertise and see a niche you start freelancing.
Dealing with mobile carriers is really hard - their tech stacks are awful and outdated and they see everything as a threat and try to protect their turf by any means.
Source: I'm trying to launch an MVNO and struggling to find a host MNO/carrier.