Android and ssl from Let's Encrypt?

Question

Android

Installed SSL on the site from Let's Encrypt.
The problem is visible so far only in Android browsers, everything is ok on desktops.
In a mobile browser, I can not access the site due to an error.
OpenSSL> s_client -connectgives an error message:

CONNECTED(00000003)
depth=3 C = SE, O = AddTrust AB, OU = AddTrust External TTP Network, CN = AddTrust External CA Root
verify error:num=19:self signed certificate in certificate chain
verify return:0

Those. one of the certificates in the chain is self-signed.
Firstly, I sin on the order of inserting certificates into the hosting panel (Reg.ru ISPmanager). The part recommends to put fullchain.pem instead of cert.pem - done does not help.
Where can I get the correct chains of certificates, tk. None of them are accepted by me as a chain through the hosting panel / adding an SSL certificate?
What to write in the certificate and chain fields - please explain on the names of the files issued by Let's Encrypt (cert.pem, chain.pem, fullchain.pem, privkey.pem)

Answer 1

What Android do you have?