Please fill out the fields below so we can help you better. Note: you must provide your domain name to get help. Domain names for issued certificates are all made public in Certificate Transparency logs (e.g. crt.sh | example.com), so withholding your domain name here does not increase secrecy, but only makes it harder for us to provide help.
You had a certificate already from Let's Encrypt that did not cost anything. I don't know why you would have then purchased one from them.
Yes You need a virtual host for port 443 and its SSL configuration. You can use the below site for help. I do not recommend enabling OCSP or HSTS Strict Transport until you know for sure what those do they are very advanced features which can cause problems.
If you want to continue to use your SSL Trust purchased certificate, I recommend getting in touch with SSL Trust with regard to the further use of that certificate.