A bad route would be the first thing I’d check, too.
It sounds like u/[email protected] is already pretty familiar with the Linux command line, but just in case, you can check the routing table (on both the NAS and the client machine) with ip route (that should show the whole routing table) and get the specific route to the remote device with ip route get 10.42.69.XXX (change the XXX to make it the address of the remote system). If either side shows that the route is going over your default gateway, that’s your problem.