have you thought about writing a simple script with ifconfig <WAN interface name> down and another one for up and add it to your CRON jobs?
igb0
ath0_wlan2
ifconfig ath0_wlan2 down
That's it, it should all work. Try and understand what's happening rather than just copy and paste, it will prove beneficial down the line if you understand what's going on.
2020-09-18T11:50:00 configd.py[32149] [83286599-3f93-4a5c-895d-149fcb25ad83] returned exit status 1272020-09-18T11:50:00 configd.py[32149] [83286599-3f93-4a5c-895d-149fcb25ad83] Disabling Interface
2020-09-18T14:14:32 configd.py[32149] [37f6a30b-8b51-41f0-8de3-fd7f528db74f] restarting cron2020-09-18T14:14:32 configd.py[32149] OPNsense/Cron generated //var/cron/tabs/nobody2020-09-18T14:14:32 configd.py[32149] generate template container OPNsense/Cron
2020-09-18T14:16:00 configd.py[32149] [9945d40d-1b08-47c1-b793-200f0cac8655] returned exit status 1272020-09-18T14:16:00 configd.py[32149] [9945d40d-1b08-47c1-b793-200f0cac8655] Disabling Interface
the error you are seeing would suggest you haven't set the executable flags on the sh file, so check that.
drwxr-xr-x 2 root wheel 1024 Sep 18 10:42 rc.d
-rwxr-xr-x 1 root wheel 220 Sep 18 10:17 Interface_up_down.sh
# /usr/local/etc/rc.d/interface_up_down.sh down-sh: /usr/local/etc/rc.d/interface_up_down.sh: not found
2020-09-18T19:45:00 configd.py[35743] [c0de5caa-ec09-4a6a-bc82-d7d9933daf0c] returned exit status 127
# configctl interface_up_down downError (127)