I am with Spark, on Fibre Broadband, Using standard SPARK HG659b router. And Synology NAS 415play.
Firmware on router: V100R001C227B022
NAS is fully up to date.
I have in the recent past (a few months ago) had no issues connecting to the NAS from outside the LAN on:
port 5001 to access the NAS Diskstation Manager
port 1194 for VPN
port (non-standard #) for SFTP
I have not changed anything in recent times. But I can no longer access any of the above.
I have checked the port forwarding rules in the router, and they all look good to me. No apparent change.
I have checked the firewall in the NAS, and that also looks good to me. No apparent change.
I restarted both the router and the NAS
As far as I can tell, I should be able to connect, but I can't.
 I can connect via the Synology "quickconnect.to/quickconnectID", which I believe uses 80, 443, and/or 8888.
I don't know enough to understand where the block might be happening. Any ideas how I can work out where the problem might be happening?