If you set up the primary node to send its log to an rsyslog server, then the remote nodes will do so as well. (At least, this is what I've found with an all-XT8 mesh.) Fill in an IP address in the "Remote Log Server" field in the System Log tab. It's not too hard to configure syslogd on a Linux machine to record traffic from the nodes.
When you're using wireless backhaul, it might be that log messages that need to traverse the backhaul link get dropped just when you need them most. So ideally you'd make sure that the rsyslog server has a wired connection to whichever node you're most worried about seeing the log of.