Hope this info helps someone.
Today was one of those days when "something" crashed the home network... No traffic/UTM stopped responding etc
Restarted the UTM and the Mesh/ONT routers too as it seemed like a good idea.
Unfortunately the once the UTM was up the WAN was State=Down... figured the fibre router had possibly updated/reset to default, possibly a MAC address change etc etc...
Everything was able to see each other, ping's working fine, MAC fine and the routers working fine with a connection/DNS etc and serving websites on a direct connect to the fibre router.
The UTM External WAN Interface was not responding to "Renew" and any minor edits to the object came back with an error.
A search on the forums suggested deleting the External Interface and recreating it... NOT A GOOD IDEA and a last resort. Yes sure if its a new install, but if this is your hard baked system, you will need to reconfigure and recreate any instance of the Interface use following a delete...
Fix: Management > Backup/Restore ... pick the most recent one and all should be good
I would suggest a download of few of the backups in case this doesn't work and need to rebuild