@Nossie one way to check this is to move the Pi so that it can be connected to a HDMI display and a keyboard. Next time it goes offline you can access it directly and see if it's really crashed or if it's just lost its network.
You may also find a tool like atop useful. I mentioned it in another post linked below. You can use the command atop -r to see today's log and add a space and 1 or more letter y to view yesterdays, the day before's etc.