Mounting Google Cloud Filestore to a machine on a separate network (non google), connected through ipsec tunnel
Solution 1:
You cannot access Filestore file shares from an on premise environment or directly through a VPN. Filestore is designed to mount fileshares only to Compute Engine instances.
This can be worked around by using a VM instance as a proxy.
As I understand you have already connected your on-premise environment with the VPC network that the Filestore instance is hosted and you have already tested that you are able to mount file shares on a Compute Engine VM instance.
So what is missing, is connecting your on-premises client to the Compute Engine VM instance, in the same Project and VPC network, that will act as a proxy.
You may find more details on how to configure the proxy VM and establish the connection with the remote client in Google documentation