Thank you. That's helpful.
I went back to rbfeeder.ini and changed network mode to true (since I'm now using dump1090-mutability. It seems to have somehow triggered something to make it active (hopefully)
And, just so that you know, I don't see any aircraft on the screen and there are no logs in the daily summary indicating that the feeder has become active
Besides, here's a line from the output of sudo systemctl status rbfeeder, which has me thinking:
Process: 5640 ExecStartPre=/bin/mkdir /var/run/rbfeeder (code=exited, status=1/FAILURE)