Easy way out is to assign a fixed IP to the NAS; saves you some troubleshooting time. Harder way would be to check the file services on the NAS and the Sonictransporter, both need to be capable of SMB1; I know that for a Mac running Core Windows File Sharing needs to be on but not sure for Sonictransporter - maybe this is related (go down to the part where they mention a SMB loading bug):
As I said, if it was me I’d just give the NAS a fixed IP and be done with it. Nevertheless, good luck.