My suspicion is that you'll need to forward the ports to the router from your modem.
That is to say, a request comes in on port 5000, the modem forwards it to your router, then your router forwards it to you Synology NAS.
The default web configuration IP for your modem is 192.168.1.1. Otherwise, look in your router settings to see what the "Gateway" address is. That should be the IP for your modem.