From Sept 30th 2021 Let's Encrypts previous root certificate DST Root CA X3 (and it's R3 intermediate) is expire.
As I know are plainer to support cross-sign for Let’s Encrypt’s ISRG Root X1 certificate from its DST Root CA X3 certificate until 2024 but not ready yet. Until then recommendation is for Android to install and use latest versions Firefox web browser from playstore on Android versions prior to 7.1.1. to get safety browser.