524s are covered at Community Tip - Fixing Error 524: A timeout occurred. That error is not exactly SSL related, so for starters you should leave everything at “Full strict”.
Then, if you get that error it would indicate the connection itself worked but your server did not respond within 100 seconds. If you are saying it works when you remove the network restrictions, that would suggest it actually has something to do with their firewall, maybe the initial TCP connection works but subsequent outgoing data packets are blocked. Check out the firewall settings for any of that and make sure everything in regard to these IP ranges is allowed.
If something is not clear you might want to contact your host’s support again and clarify if there is anything else you should do, you can point out that Cloudflare can connect but your server does not send anything back.