Brett,
Radon mitigation insurance is difficult to obtain since many underwriters don't want to touch it. To obtain the best rates (I could find) you need to join AARST (aarst.org), be licensed by the state you will work in or have NRSB or NEHA-NRPP mitigation certification. Since you live in IL you have to have your IEMA license before they will insure you for work in IL.

From member bennies at AARST.org
"Qualifies state-licensed and NRPP-certified AARST members with increased
and specialized Insurance that includes General Liability, Professional Liability and Pollution Liability"

I also obtain my hired and non owned auto from them at a good rate from these Insurance brokers.

Regards,
Paul Ybarra