Make sure firmware is set to Marlin, baudrate to what is set in firmware. Also select "Port is visible even when unconnected". That is a behaviour of Prusa printers that it looks like firmware is there but only the usb port which is powered over usb is visible until main power comes on. With that you get no constant switch between off/try connect.At least in MK3 you can select output port for marlin between usb port and internal serial header. If that is the same with 2.5 make sure it is selected to where you connected it.