Our last name is Fahnel which is German and I wanted to know the correct way to pronounce it. Some relatives believe it is pronounced Fennel like the spice but I believe it is either Fah-nel or Fohnel.
Assuming that the spelling was unchanged upon immigration to the US, the pronunciation would be Fah-nel (IPA: [ˈfaːnəl]), with the ah pronounced like the sound your doctor asks you to make at a check up.
That said, Fahnel isn’t an extremely common German name, and it’s very possible that your ancestors left Germany as Fähnels, and then had their name “Americanized” by removing the umlaut (see for example Müller → Muller/Muhler etc.). This would be my bet. In that case, the original pronunciation was more akin to fennel but with the first vowel being long and stressed (IPA: [ˈfɛːnəl]).
Here is a guide to the relevant IPA pronunciation for speakers of English.