Site not loading after upgrade Lightsail instance
getting 521 error. but is accessible via IP but not accessible through url
Regarding Cloudflare 521 error, may I suggest you to take a look into below article to troubleshoot the issue, therefore if you already did may I ask what have tried so far from the suggestions below?: 
Maybe something at the Lightsail firewall changed or need to be fixed?
For example, kindly re-check if Cloudflare is allowed to connect to your origin host to as follows in the below article:
Nevertheless, Cloudflare IP addresses list can be found here:
Before moving to Cloudflare, was your Website working over HTTPS connection?
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