Thanks both for your help - I couldn't find any logs unfortunately.
I've just tried factory resetting both devices (again), and repairing and it's working fine now for some strange reason!
Yes, they have unrestricted communication between each other, and I can view the live stream fine remotely via HikConnect, it's just the intercom functionality that's not working (and unlock).
Should it be protocol 1.0 or 2.0 for both?
Door Station
Indoor Station
(I have tried "Enable Protocol 1.0" = On, and Private Protocol 1.0 on both but still no joy in getting them to link)
I've had these devices for about 5 years, generally working fine. I got a new home network setup, so had to update the devices with the relevant settings, but the door station no longer links with the indoor station.
I can open the live view on the indoor station, but the intercom/unlock...