You’re sending the intermediate certificate signed by the ISRG Root X1 root certificate in your certificate chain. While this root certificate is accepted in many modern root certificate stores, it isn’t in older stores like your old Android versions.
You should use the intermediate certificate signed by the DST Root CA 3 root certificate from IdenTrust (called Let’s Encrypt Authority X3 (IdenTrust cross-signed) in the document below).
See https://letsencrypt.org/certificates/ for more information.