Server security

[castoffs]

I have a primary wireless router (dd-wrt) to which the w8 pc running the server is connected to by ethernet cable. Also connected to this primary wireless router is another wired router to which all my media devices are connected to by ethernet cable including a linn player, sat box, dvd player &tv all acessing the same w8 pc. the Linn/Minim connection is controlled by an Android tab using Bubble connected through the primary wireless router by wireless.

The problem is that anyone can connect wirelessly to the primary wireless router or to a third wireless router connected by ethernet cable to the primary wireless router. Therefore anyone can use a control app to play or download my music through Minim.