Instead, I suggest you set up Port Forwarding of all the required ports (DSM-5000/5001, SMB, HTTP, HTTPS, SSH, etc.) from WAN to LAN. Alternatively, you can put the Synology NAS in the DMZ that should forward all ports.
I doubt the GL-AR300M as a wireless repeater/repeater will provide much throughput for a NAS, especially only over the 2.4GHz band,.
I do not work for and I am not directly associated with GL.iNet