How can I change my NAT Type to Open with Fizz router? and is there anything else that can restrict the nat type outside the router?
First of all, you should typically explain the context of Open/Moderate NAT. I am assuming you are referring to the status provided by some gaming consoles/platforms ?
The issue is that by default all "home" routers will do NAT which translates the 1 public IP you get from your ISP to a way for multiple devices to communicate to the outside world. What you need to do, is figure out what ports your device is looking for, and then log into your modem/router, and add a few port forwarding rules, to send those incoming ports to the gaming device in question.
Sometimes due to the built-in firewall of the router, you will need to open ports. You may do this by either doing Port Forwarding or Port Triggering. Once the ports are successfully opened, the NAT Type will change to Open or Moderate. Thus, making the gaming console work successfully online.
For example, this is the xbox live article on which ports it expects forwarded: https://support.xbox.com/en-US/help/hardware-network/connect-network/xbox-one-nat-error
One quick option (but I don't like it) is to enable DMZ and send all incoming ports to a specific device.
Login Fizz modem menu at 192.168.0.1
username is cusadmin and password is you WiFi password.
Under the [Basic] Settings menu, you will find 2 options
1. [Port Forwarding]
2. [Port Triggering]
There is not 1 correct configuration, since the ports will vary depending on the gaming system you are using.
Doing a google search for "gamin NAT issues forwarding" and the gaming platform you are using will provide you with ample information to see which ports you need to forward.
Hope this helps.