Many people are using a NAS nowadays. Probably the most commonly used protocol is Server Message Block (SMB). This could be utilized since the XBox Media Center days. SMB is still available when adding a new source, but it is not working in Recalbox v7.0. It had been working in v6.1.1 just fine.
details:
When creating a new source, I choose SMB, but none of the sources on the network show. There are usually at least two: my NAS and my router.
When I create the source manually, I enter all details and when finished, Kodi instantly reports "software caused connection abort", therefore it seems that it is not even trying to connect. On the other hand, when my NAS is off, the error message reads something like "no route to server".
side note:
EmulationStation states in network settings that it is "not connected", even though wifi is connected and a LAN cable is plugged in. Yet Kodi tells that it is connected properly (in its network settings).
Another hint in this direction: when I manually create an SMB share, Kodi states "the connection to the network location couldn't be established. This could be due to the network not being connected. Would you like to add it anyway?"
The raspi is obviously connected to the network since it received its static DHCP IP address from the router and can play youtube videos. I can ping the raspi from my PC.
Hardware: Raspi 3b+