Also check your you diagnostic message in Linux for the USB connection: It should show the USB connection detection and assignment to the serial port, you can unplug the USB connection and plug back in to get in logs for current time. Otherwise will need to go back further in logs to find.

run at command terminal "dmesg -T" the -T option should return time in human readable format.

Example of mine:
[Thu Jun 11 09:43:46 2020] usb 1-1: new full-speed USB device number 15 using xhci_hcd
[Thu Jun 11 09:43:46 2020] usb 1-1: New USB device found, idVendor=0403, idProduct=6015, bcdDevice=10.00
[Thu Jun 11 09:43:46 2020] usb 1-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[Thu Jun 11 09:43:46 2020] usb 1-1: Product: FT230X Basic UART
[Thu Jun 11 09:43:46 2020] usb 1-1: Manufacturer: FTDI
[Thu Jun 11 09:43:46 2020] usb 1-1: SerialNumber: D306TBUW
[Thu Jun 11 09:43:46 2020] ftdi_sio 1-1:1.0: FTDI USB Serial Device converter detected
[Thu Jun 11 09:43:46 2020] usb 1-1: Detected FT-X
[Thu Jun 11 09:43:46 2020] usb 1-1: FTDI USB Serial Device converter now attached to ttyUSB1

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