May I ask what troubleshooting steps have you tried already from the article below related to your 525 error? 
May I ask what SSL option have you got selected under the SSL/TLS tab at Cloudflare dashboard for your domain ( Flexible, Full, Full Strict … )?
Before moving to Cloudflare, was your Website working over HTTPS connection?
Before doing anything at Cloudflare settings, you could determine if you have a valid SSL certificate installed at the origin host/server by your web hosting provider or your own VPS/dedicated server following the steps from below:
- Use the “Pause Cloudflare on Site” option from the Overview tab for your domain at dash.cloudflare.com .
- The link is in the lower right corner of that page.
- Give it five minutes to take effect.
- Check with your hosting provider / cPanel AutoSSL / ACME.sh / Certbot / Let’s Encrypt or some other and renew it accordingly.
- Make sure site is working as expected without any errors via HTTPS.
- Only then should you un-pause Cloudflare and double-check your SSL/TLS setting to make sure it’s Full (Strict).
Here is a way to re-check if you correctly setup the SSL for your domain with Cloudflare: