My suggestion is to take a step back and reconsider everything .
First read some tutorials regarding port forwarding. For example:
https://www.lifewire.com/how-to-port-forward-4163829
https://www.noip.com/support/knowledgebase/general-port-forwarding-guide/
Be sure that you understand the overall concepts and what is required.
Second go online and look for the applicable User Guides/Manuals for your modem and router.
After reading the tutorials, the information and instructions in the User Guides should be much more understandable.
Verify that only one device is assigning DHCP IP addresses. Router recommended - not modem.
Ensure no duplicate IP addresses exist.
Third plan your port forwarding configuration. Sketch out a network diagram labeling all devices and drawing in the connections between devices. Keep the diagram simple but detailed enough that anyone looking at the diagram will have a sense of the overall network layout.
Be continually sure that everything else (your network, your internet connectivity) is working as expected. All DHCP IP addresses to begin with.
Then move on to assigning a static IP to the intended target computer and, once completed, again confirm that that computer is online.
Next, set up the desired/required ports via the router and the target computer's Static IP. Be sure to have that Static IP reserved for the target computer via the target computer's MAC.
Keep notes on the configuration settings: those already set, those you set or change. Avoid multiple changes at one time. If something goes astray it will be more difficult to identify the culprit.
And pay attention to the screens: You may need to "OK" , "Save changes" or "Apply". Those ending clicks are often overlooked when things get into a rush.