port forwarding with dynanic IP addy and dyndns

Help! iI'm a fairly new to all this.

here's my situation:

I am using a DVR CCTV kit at home which works perfectly fine locally, and works externally INTERMITTENTLY - i am using the free dyndns service and use a netgear router.

I used the 'address reservation option' under 'LAN setup' and have assigned the DVR's MAC address an IP address - I assume now that the router should give this DVR the same IP address every time???

I have opened up and forwarded the ports correctly (i think) as i can access the DVR externally using my "http://dyndnsservicename.org:88"

If I set the DVR to 'get ip dynamically' the port that i have opened up works fine with the forwarding rules created.

The problem with this is that the IP address of the DVR still changes on the router every now and again?? is it not supposed to keep the same IP address for the DVR ALL the time???

whenever i set the DVR's ip address to 'STATIC' instead of dynamic so that it only has the 1 IP address - my external access stops working. i use a port checking website to test the port (canyouseeme.org) and i get a 'connection timeout error'

If i then set the IP address back to 'get dynamically' it works fine and then is accessible externally again but the IP address of the router still changes every now and again even though i've reserved the address...? I then have to go and manually update the IP address of the port forward rules and it then works fine again until the IP address changes again on the router at some point down the lime

WHAT AM I DOING WRONG?

I would have thought that by setting the DVR's ip address manually, and reserving the IP adress on the router then the device would only every connect on that IP address (which it does)- but for some reason when i do this though, the PORT FORWARD stops working; set the DVR back to dynamic and it works again; then have the intermittent issue of the IP address changing.

any help would be much appreciated.