Hey @Defit77!
1. Your issue with the cb mic might be similar to this:
In short, see if the cb mic is enabled in-game. The cb radio box should be located above the windshield in the cab. If it's on you should be able see green numbers displaying what channel your on. If they aren't on, it means it's off. To turn it on, there are three dials underneath. One small one should say on/off. Press tab then right click to display your mouse and click on that dial. The green numbers should then turn on. Also, if your testing your mic by trying to talk with a friend, make sure that your both on the same channel. You can change the channel number by clicking on the big dial next to the smaller dials.
2. Please take a look at this knowledge base article to make sure that you have installed it correctly. Something may have gone wrong in the installation that would make TruckersMP not recognize it. https://truckersmp.com/knowledge-base/article/1122
Hope this helps!
Farm Shark