You need to generate newly purchased SSL certificate, then in cPanel you should upload and activate the certificate. If you buy from Unitech, submit a support ticket we will do the rest for you.
Usually Name Server or DNS changes take time(24 to 48 hours) to update in all Internet Service Provider. Check update status, instantly from Unitech DNS Checker.
In this case, you need a DNS hosting, for example: Use CloudFlare, add your domain. Get the NS Records, update that to your Domain in Unitech Client Area.