DIY Radon Mitigation Strategies?

I recently moved to a new house that has a radon mitigation system that was poorly installed. For example there was no pit underneath the foundation just some gravel. I know this because I had a plumbing issue and had to rip up the floor. The system is still running, as well as a ventilation fan to clear the air by the boiler but my numbers in my basement are still generally above 4.0. I don’t want to pay someone to fix this because I’m pretty handy and want to fix it myself. Has anyone done this or have any pointers for what I can look at to learn more about effective ways to mitigate radon in an older home?