Feeling like stuck in a wrong field of study due to inept technical background. Seeking advice on how to sustain in academia and research? I completed my PhD from North America recently in a particular subfield in mechanical engineering. Though, I have published 3 decent publications and got a postdoctoral position afterwards, I am feeling lost about my ability to contribute well in my field of research.

The reason being that my research area is interdisciplinary and researchers from different fields like physics, computer science, electrical engineering, materials science, mechanical engineering, civil engineering work in this "hot" area. My background is materials science.

Mathematical expertise ( mainly Calculus), programming proficiency (python) and knowledge of statistical methods (for deep learning applications) is a huge advantage in my field, which I lack due to my poor mathematical background. My undergraduate mathematics grades were all B- and C. Didn't take any grad level mathematics courses.

I am self learning all the advanced concepts and relearning the fundamentals, but I feel that I will always be at disadvantage from my more fundamentally sound counterparts.

I am 30 now, want to give my best shot at academia for the next 3 years. I feel like I am stuck in a wrong field because of my poor fundamentals and knowledge. Any advice on thriving in such a scenario? I enjoy my research field, but being less technically sound makes me question my place.
I think I know what you're going through, since I'm also from a materials background and need to work with teams comprising mechanical engineers/physicists on mostly inter-disciplinary topics. Through some limited experience with your specified sub-field, I am quite confident that materials science by itself has very unique and valid contributions to make, even without the mathematical/statistical tools that you mentioned. (Those tools, incidentally would be great assets in almost any field, IMO).

The key fact is, on such teams it is necessary to speak the same language. You must pick up on the shared jargon and to resist the temptation to view everything through the lens of individual specialisation. This will force you to acquire a basic familiarity with other team-member's work. Once you acquire that and have a generally pleasing personality, you will likely find a lot of help and resources coming your way. In some sense, your team-members will become force-multipliers, and it will get increasingly easier to learn the ropes and find a niche where you can contribute uniquely. This may all take some time, so don't worry if there are some lows along the way.

Even while learning new things (it goes without saying that you must be open to this!), don't forget your primary specialisation/skillset, and look for problems that can be solved using that. If they aren't any obvious problems, look for places where you can help to optimise or enhance something. If even that doesn't work, start off by considering parametric studies, where you vary certain parameters unique to your specialisation and see the effect it has on the overall process. This will also accelerate your learning and help you get a feel for the field. Naturally, you will need to convince others that such studies are reasonable, so adequate background work and literature surveys will be required. 

Finally, don't overlook the personality aspect, because you would like people to work with you, not against you.