Pursued PhD under an advisor who did not know what to do. Now am stuck with future prospects I made a mistake 5 years ago. I changed my field from materials science to civil engineering. I did a PhD under a professor whose research expertise was not in my research topic. I had to independently work on my knowledge and skill set to become productive and produce research. But, due to lack of technical guidance, I produced mediocre research. My research is very novel and rigorous for my field of research, but the scope of getting a future position in the field seems bleak.

I want to diversify to other area of research, but I don't possess the experience or knowledge to carry out the projects. I am getting rejections from all postdoc positions as I don't meet their requirements.

I don't know what I should be doing now? My adviser doesn't have any connections in my research area. She just says to keep trying, and to not lose heart.
First of all note that your experience has probably made you a lot more independent than many of your peers. This is a desirable quality for a post-doc, and you should sell it. If you have produced any a single author  publications (which sounds likely in your scenario), this is again something you should emphasize.

As a second piece of advice, consider applying for a suitable post-doc fellowship. Advertised post-doc positions are often bound to specific funded projects, therefore a PI will be more inclined to hire somebody with proven expertise to work on that project, putting somebody switching (sub)fields after their PhD at disadvantage.

Postdoctoral fellowship applications allow you to put more emphasis on your individual qualities and allow you to tailor a research proposal to your combination of expertise. You would (in most cases) still need to find a suitable host for your fellowship, but this may be easier than getting them to hire you as a postdoc (it is relatively low risk for the host). Of course, you should be warned that many fellowships have a very low acceptance rate. (10% acceptance rate is not uncommon for many grants, although some have rates as high as 30%) So, you need to submit a very strong application.