I am frustrated by my inability to solve what I know is a simple problem

I have a vigor 2860n - gateway address 192.168.1.1
I have a network video recorder on the Lan address 192.168.1.20
I usually access the nvr directly from my browser by its lan address.
I now want to access it via the internet .
The NVR has a server port 8999 and http port 80
I have opened port 8999 on my vigor in the same way I have for other devices on the lan (network cameras etc)
A port scan shows the 8999 port to be open.
When I try to access the nvr by my public IP address xx.177.xxx.x38:8999 I am getting this page cannot be displayed error.
Each of my other attached devices is reachable ok but not the nvr.
Anyone got any suggestions as to how to track down a solution?
Frustrated
Graham

Not sure what the difference is between "server port" and http port but have you tried changing the default router http port from 80 to something else, such as 8080?

I have tried setting the default nvr http port to 8080 leaving the vigor as 80 and the vigor as 8080 and the nvr as 80, but no luck.
The odd thing is that each of the other devices attached to the network works ok.
Eg I have an axis camera server 192.168.1.70 with a server port 8090 , I type my public IP address xx.177.xxx.x38:8090 and get thru straight away, that also has an http port of 80?
I thought that port 80 gets me to the router and then say 8999 gets me thru to the nvr. Its most strange. Ive done the usual things , turned off firewall etc but to no avail.
If I hit public IP address xx.177.xxx.x38 with no port specified I reach the vigor router login page, but when I append :8999 it wont pass it on to the nvr login page.
:?:

problem now solved, had set http port on nvr 8080 but needed to reboot nvr to make it effective.!
thanks for help