On your PS5:

Go to Settings → Network → View Connection Status and write down the IP address of your PS5

On the SURFboard Central App:

Go to Network on the bottom of the screen and expand the Network Settings Section and select General Settings

Select Port Forwarding

Select Add Rule

Select TCP/UDP on the top of the screen and enter 3478 in Internal Port Start and External Port Start.  Enter 3480 in Internal Port End and External Port End.  Enter the IP address of the PS5 you copied down in the first step.  

Repeat the process and add two more rules for:

TCP port 1935

UDP port 3074