DNS updated, SSL installed, site still shows "Not secure"

Before answering the above concern, we need to understand what is a Browser DNS cache, OS DNS cache, ISP DNS cache

DNS caching involves your Operating System (OS) or browser capturing recently-visited IP addresses and saving them in a database. This enables your browser to reload sites you’ve visited in the past more quickly than if it had to reference the DNS again each time. It can also become outdated or create internet connectivity issues. In any of these situations, clearing your DNS cache can often help you resolve the problem and get back online.

Clearing OS DNS Cache: https://kinsta.com/knowledgebase/flush-dns/#an-introduction-to-dns-caching

Clearing Browser DNS Cache: https://torguard.net/knowledgebase.php?action=displayarticle&id=230

Clearing ISP DNS Cache: ISP's DNS cache is something that cannot be controlled by the users. The DNS of ISP is refreshed only once the TTL set for the DNS records is expired. ISP DNS can be bypassed but since that is something technical, we do not recommend it to our users.

You can refer to this link for resolving the "Not secure" issue: 

https://secure.helpscout.net/docs/5ec237862c7d3a5ea54b448d/article/60beff666264f06fc02feccb

If the solutions mentioned in the above link don't help in resolving the issue, change the network you are connected to and try using the home network or different Wifi network and the issue should be resolved.