Comodo certificate

Hello, i have an Comodo free certificate and i have the four files. I installed the my-domain_com.crt certificate in cpanel but now, only on my samsung Phone, the Website says: Not Secure! I read i have to install the addexternaltrustrootca.crt but how?

You can’t install the CA chain here, you need to upgrade your account to do that.

I cant upgrade my account with my paypal account