So as an 'IT guy' I've had to replace routers where for many different reasons being able to login and look at the original settings isn't possible. In these cases what I do is run an IP and port scan of the most common subnets and then investigate which services are actually used by temporarily inserting my laptop as a man in the middle and capturing the packets while certain activities are triggered (flipping switches, blinds, etc), and then I have a pretty good idea of what needs to talk to what and how. It's easy to setup a new router with all essential port forwards and settings after you have a proper map of the network, unfortunately most IT guys will just assume things and not be that thorough..