Error 502 after enable cloudflare

Hello everyone,
I’m new in cloudflare. Yesterday I upgraded my host plan. I also remove and add the domain because the DNS records but now my whebsite is showing Error 502 Bad Gateway (Screenshot attached).
I already talked to the host providers and they say it’s all OK on their side.
They asked me what ips cloudflare uses to add to their firewall.

After they add all IP’s to whitelist nothing changes. The website continues to display the error.
What kind of tests I can do?

First and foremost, a 502 is covered at Community Tip - Fixing Error 502 / 504: Bad Gateway

I already try everything

What did #2 reveal?

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