Not sure if other people have had this problem but sometimes when I plug my devices in they do not always get the same ttyUSBnnn number..
One solution is to simply go into into indi settings and change to the correct number(messy)., here is another.

In /etc/udev/rules.d I have a text file called 82-usb-name.rules
In this file I have
# Allocate permanent name to ttyUSB0/1

KERNEL=="ttyUSB?", ATTRS{idVendor}=="0711", ATTRS{idProduct}=="0230", ATTRS{serial}=="097876", SYMLINK+="filter", MODE="0750", GROUP="dialout"
KERNEL=="ttyUSB?", ATTRS{idVendor}=="0403", ATTRS{idProduct}=="6001", ATTRS{serial}=="FTE19QEG", SYMLINK+="focuser", MODE="0750", GROUP="dialout"
NOTE IN THE ABOVE I ALSO have a serial number specified. So if you have more than one device you separate them.
What this does is pick up any USB device that matches the IDVENDOR and IDPROUCT attributes and gives it an apropriate symlink name. So in my case I have a filter wheel and a tcfs focuser which get called filter/focuser. Then when everything is powered up you do a ls /dev/f* both names show up.
If they do not show up you probably have the product/vendorid wrong.
So now in my indi settings I set the filter wheel device to /dev/filter and the focuser device to /dev/focuser and can connect with no problems. I do not need to know what the actual usb device number is.
CAUTION: if the device names have shown up but you cannot connect it will not be a udev rules problem but a permissions problem.
Hope this helps, Brian

Thanks for sharing this Brian. I have two devices (scope and focuser) using the same VID:PID so they get assigned different ttyUSBX every time and it's trial and error to check which is which, now I could use the serial attribute to distinguish the two!

Hello,
Another potential problem is different software authors installing a rules file for the same device. By not deciding on a single rules file name, it is possible for the rules of one software package to interfere with another. An example of this is the QHY camera rules file. Depending on the driver being loaded, the device final address might not be the same.