My Synology is on LAN but not connected to internet

I am a bit at a loss what to do next.

2 days ago I got some notifications that Synology could not reach my box. When home I began checking and in fact most things could not find the box. (though fascinatingly SMB could access the box - Lightroom was happily seeing its files on the NAS).

Then I used Synology assistant and found the box (with a new ip ..ugh). But even when I connected the box is not connecting to the internet. I can access it via WIFI on my LAN .. I see the router sees the box, but I cannot connect to the internet.

(I attempted restoring network settings from backup, I rebooted router and box .. no change)

Has anyone seen anything like this before?

Is there any good guide how to re-set and set up network settings and external access ?

If I need to restore to factory settings what is the best way to get back up ad running after? and not lose my data?