Remote access with port issue

[SparksTang 0]

Hello,

I'm new to emby. And I got a issue with remote access to emby server. 

I have emby 4.4.3.0 installed on Synology NAS (1819+). Router: tp-link 8600. I set the port as default 8096 and internal connection is good. But when I try to access from external, it doesn't work, neither with IP nor with DDNS. The port forwarding is set and working well for other application such as transmission, webDAV or synology quickconnect, and even later for jellyfin. 

Then I tried to use different port. First I tried to change both the local port and the public port. And I got the external connection. But second day the new port doesn't work again. And then I tried to only change the public port and keep the local port at 8096 with the port forwarding setting accordingly: public port -> 8096. And it worked again, but not for long. A moment later, it doesn't work again. And I tried again changed public port and worked again and stopped working again later. It seems the port set in the emby always get blocked somewhere somehow. I checked firewall setting in Synology and also tp-link router. Didn't find anything. 

Anyone had experience with this kind of situation? Any hint where could went wrong?

Thank you in advance

Sparks

[jch 2]

What do you mean by your “public port”?   What URL are you attempting to use to access your Emby server remotely?  Something like  http://your.external_IP.address:8096 ?  It sounds like you’re not configuring your router correctly.  You need to open port 8096 on your router so that it can be accessed externally and then forward that port internally to the local IP address of your Synology NAS.  Are you doing that?  I cant think of any reason your access would come and go as you report.  QuickConnect doesn’t require port forwarding by the way.  Do you have the firewall on your DS1819+ turned on.  How is that configured?

[SparksTang 0]

Your understanding on public port is correct. I think I set port forwarding correctly otherwise I won’t be able to access Emby externally even with this short moment. 
I have firewall of NAS on. I checked the setting, nothing suspicious. And I tried turn it off. But I still could not access the port when it’s off. 
I suspect more on router firewall. But with TP-LINK UI, I can not find anything.

[jch 2]

Have you assigned your NAS a static IP address?

[SparksTang 0]

Yes, assigned.

BTW, I will change a new router, see whether it helps