It seems my problems with connecting externally are fixed when, on a new install, I set up Quickconnect. That at least allowed Synolgy in to troubleshoot, but I'm a bit sketchy as to how it actually fixes outside of LAN availability.
Spoke too soon and have had to open up the ticket again with Synology. NAS can't see externally again, connection failed when looking at packages etc. All I did was change the internal ip address to what it used to be before the last NAS was RMA. Nothing else has changed. Have rejoined domain fine, no domain/ldap errors on the status check. The only clue is that Quickconnect, although ticked as it was prior to the ip address change, now says:
Network Error occurred. PLease check your DNS and network settings
Can't see where it is going wrong unless the DSM doesn't like changing from the initial setup.
No changes, gateway is the normal IP for the router, DNS points to the same DNS server as it has done since I've had a NAS. Can't change DNS server to 184.108.40.206 as some similar threads have suggested as it is joined to a domain so the only DNS server I can change is the aforementioned domain server which will kill off internal DNS for the NAS. Very odd...
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I just flipped it back to the default IP and then flipped it back again to what it was on the last post and now it can see the external network....go figure...not sure what it's not liking about IP changes to randomly work/not work but I'm going to keep a eye on it