I know nothing about NAPT (Network Address and Port Translation) & how it compared to NAT (Network Address Translation). So I needed to investigate the difference. The difference between nat and napt is that NAT is based on IP and NAPT is based on port. Difference between NAT & NAPT

I am not sure why in step 10 of 16 of your instructions they recommend disabling UPnP. I'd suggest you try enabling it to see if it fixes the problem. Remember you will need to save the settings & restart the device/connection.
UPnP allows LW (FW) to communicate directly with the device to open the ports it requires. With UPnP disabled, I'm not sure whether NAPT will act as a firewall or not, & if yes, FW would probably not be able to find an open port ... at least as it is set at since it's UPnP messages would be ignored.