Hi, I've always found Synology NFS at bit flaky, so my Synology is connected to Kodi via SMB. I believe in the add / browse option the link is :
SMB://<NAS hostname or IP address>/NAS_Shared_Folder
Then just give Kodi the security credentials if they are et up.
You do need to enable SMB in the Synology DSM under Control Panel >> File services (same place as NFS enablement).
Best of luck.