It limiting you to 8 ports... hmmm... Id get a different router that allowed for more. As for the only way to get around this 8 port limitation would be to open it wide open with a port range and create a security risk of ports not necessary to to be open for targeted computer at that IP address. The dynamic port forwarding from the looks of it is the only one that allows for ranges to be set. Ive never gone dynamic with any of my routers as for i feel more secure with static values of strict permission. Dynamic to me is too lax of strictness. It will allow looser security. You could get it working, but at a cost of lesser security to systems within your network.
One way to force the computer to always be at a specific IP would be to set the IP address on it statically to say 192.168.1.251. Then alter your DHCP setting for your router to only offer 192.168.1.2 through 192.168.1.250 as part of the DHCP IP pool, and this way no dynamically IP given device will ever be the same IP as this system at 192.168.1.251 as well as yoru computer will even work without the need of DHCP. To do this you go into the network settings for the network adapter and select Use the Following IP Address and then enter 192.168.1.251, then your subnet mask the same as the subnet mask you had when you were connected to router through DHCP given IP address, and use same gateway as that of DHCP issued addresses which is usually 192.168.1.1. You then will want to also set up the DNS with static values as well. Before going in and messing with values you can write down the info from running cmd at start run and the enter IPCONFIG/ALL and press the enter key. You will see an IPv4 address, subnet, gateway, and DNS IP addresses. Write these down. Then go into the adapter properties for the NIC and specify the static information where you will be entering everything you wrote down the same in appropriate fields except for that your IP Address you want to be 192.168.1.251
Remember you need to go in and change the DHCP range to end at x.x.x.250 or else another device could be given x.x.x.251 and have duplicate IP conflict.
There is the local IP address which is 192.168.1.251 and if you go online to see what your IP address is, you will see it as the IP Address that is issued to you by your ISP. This IP Address is the address your modem is connected as, Your router connects to the modem and acts as a gateway to allow your own IP addresses which are in the 192.168.1.2 thru 192.168.1.254 range until the DHCP range is properly adjusted. This is why you see the 2 different IP Addresses. When connecting elsewhere to your computer that has the port forwarding you would be connecting via the IP Address that the ISP gave you, so if your address was 68.103.74.128 and you wanted to connect on port 443 for a file transfer it would look like 68.103.74.128:443 where the colon specifies that the following value is the port to pass the communications over vs standard port 80. Traffic would come into your modem as 68.103.74.128:443 and pass this to the router. The routers firewall will see traffic for port 443 and with the port forward rule would then direct the traffic to 192.168.1.251:443 and to the outside world they dont know your internal address scheme unless there is a script that scans the system that its connected to and reports back the local LAN IP address. If your IP Address issued by ISP is 68.103.74.128 then that is what hackers and others see. and whatever ports or port ranges are added to the rules, those are all the ports that a port probing would detect at your IP address of 68.103.74.128 if your ISP issued IP address that the modem was connected to is that IP address.
I dont want to steer you wrong with dynamic configuration, so maybe someone else here who uses the dynamic setup would tell you the most secure way to carry out this since your limited to just 8 ports. I use DD-WRT on my D-Link router and I havent found a limit yet and I have all sorts of ports open or on hold just needing me to select to activate them for VoIP phone, FTP, and other purposes.
Is getting a better router with better custom configuration options a choice or you have to get by with this specific router only?
EDIT: You might be able to get by more securely with this router keeping the static entries for specific ports and adding the ranged ones in the dynamic option at bottom. This might give you ability to go beyond 8 and maybe 8 static and 8 dynamic ports or ranges for 16 port rules.