This website is currently offline. refresh it

There is no issue in my server, but I often get the website is currently offline msg, try refreshing it.

… is currently offline. Cloudflare’s Always Online™ shows a snapshot of this web page from the Internet Archive’s Wayback Machine. To check for the live version, click Refresh .

How to fix this issue?

How do you know if the website is indeed online? Do you have a way of directly accessing it (ie not using Cloudflare) to check on a regular basis?

I use third party tools to check direct access such as uptimerobot but it does require a direct connection.

The server is managed by me.
Do you have any tool suggestions to check server uptime?
I think it may be cache issue.

I use digitalocean server monitor, I did not get any such issue. I am also using Cloudflare WordPress plan.