No access to console. No cable.
Normally, you don't need a special cable.
First thing to do when you take possession a 'system' : (whatever system : your router, the fridge, tv-set, etc etc ) : activate ssh access. Because this is the 'real' admin interface ;)
Other explication : when you modify something on your main door, check-out FIRST your secret back-door (the same ssh access).
Btw : when you use something like this https://www.netgate.com/products/sg-1000.html I wouldn't use it before i has this special cable …. (locking myself out, well .... I know .... seen that - been there)