Depending on how curl is checking the trust store for root certificates it could just be that ISRG Root X1 isn't found in under Certificate > Local Computer > Trusted Root Certification Authorities. If not, try something like this to install it: Subset of Windows Users See Expired Let's Encrypt Certificates with `curl` - #24 by webprofusion
Assuming curl is present on the machine curl https://acme-v02.api.letsencrypt.org/directory should return a response OK. If the ISRG Root X1 cert is missing, try curl before and after installing and let us know if that fixed it.