To be clear, your 4 other devices are online? Are you flashing the same image in the SD card as the previous ones? What Raspberry Pi version are you using? (PiZero/1/2/3/3B+?). What OS version is it?
Could debug things as:
- check that the image is appropriate for the given Pi version
- connect the device with an Ethernet cable, which should not rely on wifi, if that is an issue
- can flash a development version image, and then you can either use a USB-to-UART cable, or this PiUART to connect to the device over serial, and see any logs there
These are some ideas, let us know if you have a chance to try things!