If you can have a fixed IP for your cloud application, you could setup a collector that is only reachable from that IP and work that way
If you have applications in IaaS/PaaS (AWS, GCP, Azure), you can setup a collector in that environment without exposing it to the internet and just allow the traffic “internally” through vnet, security groups, etc.
You could setup a VPN connection between your cloud environment and you existing infrastructure that hosts your collector
Some solutions offer export to S3 and you could ingest that from a collector as well (I don’t believe Blobs are supported at the moment) - that would not be syslog though