As a new faculty, I lack the knowledge of fundamentals in my field. What should be the course of action under my special circumstance?
You indeed need to master the topic that you will teach. So before each lecture, you need to make sure that you master the content to be able to teach it and to answer potential questions from students.
Having said that, you still have some control over the content that you teach during each lecture and how you teach it. Thus, you can sometimes select some parts that you prefer to teach over others. If you use some good textbook, then your job will be much easier.  So looking for a good textbook or teaching content online can help. And you only need to master the parts that you will teach.
Normally, you should be able to prepare 2 hours of lecture in 1 day or less, even if you are not very familiar with the topic. But if you have a good textbook it can be easier. So if you have one month, you have a lot of time in front of you.
Having said that, the life of a new faculty member is generally hard the first one or two years because of all the new courses that we usually have to teach. But then it gets easier as you teach the same courses again.
When I started to teach, I had to teach three new courses on a 1 week notice, including one that I was not familiar about computer networks. That semester, I worked hard everyday from morning to evening and took a one month online course at the same time to master the topic that I would teach the next semester. Then, after the first year, things got easier, and I focused more on research again.