Which is weird, since if your system is up-to-date on patches it should be having its trust store updated automatically. There is some lazy-loading done by Windows, but visiting an ISRG Root X1 site (like the API site, or like https://valid-isrgrootx1.letsencrypt.org/) from IE/Edge should load it and "just work".
There's a patch I've seen referenced here before to help gets Windows systems up to date, at https://support.microsoft.com/en-us/topic/support-for-urgent-trusted-root-updates-for-windows-root-certificate-program-in-windows-a4ac4d6c-7c62-3b6e-dfd2-377982bf3ea5, but I don't see anything there for Windows 2019. (Presumably because that's new enough to not need it?) But maybe that gives you a starting point to start digging from?