DropBox is free and works seamlessly OK you only get 2GB storage free but that can quite easily be increased to 20GB.It works for me anyway.Plus I have 1TB NAS Drive soon to be increased to 2TB. I use Goodsync to back up selected folders to the NAS drive and as any files are added or deleted these will also be added or deleted from the NAS drive. I have only 3 PC's connected and I must admit it works pretty much unattended. I check now and again that the folders have been kept in sync and so far so good.
OnePlus 5T review: Hands-on