The error "Connect timed out" isn't directly related to the certificate, it indicates that your client got tired of waiting for the server to respond and gave up. Important note: the client and the server can be the same program if Confluence tries to "talk to itself" through the REST API, which Jira Datacenter certainly does.
This can be caused by anything interfering with the delivery of network traffic in either direction. Common reasons are network or system firewalls filtering traffic, misconfigured NAT/PAT translations, or the interface receiving traffic for 35.155.221.139 being down, even if other interfaces on the system are up. You'll need to talk to a networking person to track that down.
It's possible that a web application firewall (WAF) is trying to connect to your server and is failing to connect upstream because it doesn't trust the certificate you have loaded, so it doesn't respond to anything on that IP. It's incorrect, but sadly common, for people to trust the "end node" certificate when trying to solve certificate trust issues when they should trust a signing CA.
I can give you more specific ideas if you paint a clearer picture of how traffic reaches your system from the Internet.
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