Hi. It sounds like you are doing FM peer to peer sharing. There are some limitations and warnings from FM re: peer to peer hosting across remote networks: Setting up a Computer to Share Files with FileMaker Pro | FileMaker
You might be getting to the stage where you should consider using FMServer. This is designed for the sort of usage you are talking about and has the added benefit of being built to backup correctly without issues of corruption etc. I would strongly encourage you to consider moving to FMServer.
In particular note this comment from the link:
To prevent others from attempting to directly open a hosted file across the network using the file sharing capabilities of the computer's operating system, it is recommended that file sharing be disabled on the host machine. This includes storing your FileMaker files in a shared directory. Attempting to share access to a FileMaket Pro database using the file sharing services of the host's operating system puts your FileMaker file(s) at risk for file corruption.