How to install R11 certificate manually on Android

[nimon]

I use some website daily that was signed by R11 certificate. But R11 certificate is not installed on my Android device. So I get the error "Connection is not secure":

""Your connection is not private
Attackers might be trying to steal your information from wpoatx.com (for example, passwords, messages, or credit cards). Learn more
NET::ERR_CERT_AUTHORITY_INVALID

This server could not prove that it is wpoatx.com; its security certificate is not trusted by your device's operating system. This may be caused by a misconfiguration or an attacker intercepting your connection.""

It will be very helpful if you can provide the certificate file of R11 Certificate file so that I can install it on my Android device manually. (Its an old Android 6 device.)

[orangepizza]

you'd want to install ISRG X1 and X2 from Chains of Trust - Let's Encrypt

[nimon]

Thanks installing only ISRG X1 certificate. solved the problem on every site uses Lets Encrypt ssl certificate.

File link: https://letsencrypt.org/certs/isrgrootx1.pem

[aarongable]

You'll want to install ISRG Root X2 as well, because some sites rely on that one.

[Osiris]

You probably already know, but for the sake of completeness: if you're still running such old Android systems, you might want to consider updating Android if possible due to lots of unfixed vulnerabilities in these older, end-of-life software versions.