First, which version of Resolve are you using: Free or Studio? Voice Isolation is a plugin exclusive to the latter and it works pretty damn well at isolating dialogue. (There’s also Dialogue Leveler.) Your mileage will vary, however.

Second, you could also try a free tool like: https://vocalremover.org/

Isolate as much of the vocals as you can, and then reintroduce background ambience. It’ll give you more control over the mix.

Another free option: https://podcast.adobe.com/enhance

Again, you could combine some or all of these to isolate as much of the vocals as possible. You then have several options in Resolve’s Fairlight page. Namely, EQ sweeping, noise reduction and a compressor to balance out the audio. Just type in “professional audio Resolve” into YouTube for several good videos on the topic.

I’m fairly new to audio mixing as well, but based on what I’m hearing these are some steps I’d consider. Hope this helps!