You need to upgrade your Operating System to have HTTPS properly displayed on your browser
Issues with
OpenSSL <= 1.0.2 Windows < XP SP3 macOS < 10.12.1 iOS < 10 (iPhone 5 is the lowest model that can get to iOS 10) Android < 7.1.1 (but >= 2.3.6 will work if served ISRG Root X1 cross-sign)
Mozilla Firefox < 50
Ubuntu < 16.04
Debian < 8
Java 8 < 8u141
Java 7 < 7u151
NSS < 3.26
Amazon FireOS (Silk Browser)
Please use Firefox browser if you are unable to update your OS & browser
Refer
https://techcrunch.com/2021/09/21/lets-encrypt-root-expiry/