I am no expert on File and Printer sharing because I never use it so I am afraid I cannot help you there. But I think you should consider not using it either.
Instead, that ASUS RT-A3200 is a fully capable router that supports network printing. There are many advantages to network printing so I highly recommend connecting your printer directly the router. In this way, all your networked computers can print without the need for the printer to be tethered to a host computer that must remain up and running, and without the need to open up printer sharing on that computer. The printer can be centrally located (off a users desk) anywhere network access is available.
If you need file sharing, I suggest a
NAS (network attached storage) device. With a NAS, you (as the admin) control who has access to the device and from there, you and whoever you allow can access (share, edit, save, etc.) files stored on that device. If you get one with lots of drive space, the NAS can even be used to backup your computers and important files - a very good thing! And this NAS device can also be located anywhere network access is available. Including hidden in a closet out of sight of a burglar.
The point about using a networked printer and NAS is sharing does not need to be enabled on any computer so it makes security much tighter. And the host computer does not need to be up and running for others to access the files either.
If you want to continue using File and Printer sharing, a bit more information is needed. Does sharing become disabled on both computers or just the one? When you say the files are visible but not accessible, do you get an error message when trying to access?