Port Forwarding Problems on Netgear Nighthawk R6700

I've been trying to forward ports on my Netgear R6700. V1.0.1.16_10.0.30

I have an Arris 6183 modem.

After playing around for several hours last night I managed to get it to work, but I had to check "Default DMZ Server" and then manually set an IP address. However, when I turned my computers on tonight when I got home from work, none of them could connect to the internet. I could not even connect to the router with the 192.168.1.1 login. I wound up having to reset my router completely to reestablish internet service. I'm now afraid to select "Default DMZ Server" box again.

One of the problems I seem to be having is that my 3 computers are randomly assigned IP addresses each time I turn them on. I need these ports specifically for a program I run on my laptop computer.

The ports I'm trying to open are 1700 and 1800.

NAT Filtering = Open
UPNP = On

Under Advanced I've selected Port Forwarding tab. I have the box checked for Port Forwarding (not port triggering) and typed in 1700 for external and internal. And 1800 for external and internal. For Internal IP address I chose 192.168.1.2 because that's the IPv4 address that was assigned to my laptop last night.

One of the problems I seem to be having is that my computers get randomly assigned IP addresses each time I turn them on.They vary between 192.168.1.2, 192.168.1.3 and 192.168.1.4

I've tried testing the ports on canyouseeme.org but I always get the fail message.

Any help would be appreciated.