Hey,
Right now, our free cluster type supports connecting using TLS only and requires your client to support TLS SNI.
Additionally, without DNS resolution, your client will not be able to perform hostname verification either, which will make using TLS even more complicated.
Maybe you could try whether it will work if you assign a static hostname to the cluster URI’s IP address on your device somehow? We cannot currently guarantee that the IP of your cluster address will not change though, it might be useful for testing though for now. (Please report back whether this works for you)
What type of device/client are you using?