I've also been able to open my NAT now. It doesn't help with lag, but it opened my NAT. I was running two routers at the time with the NetDuma being the secondary.
I removed the primary router, and now the NetDuma is my only router. I did two things. Bare in mind I was frustrated at the time so I didn't try anything else.
I removed the primary router. I double checked my download and upload speeds in the NetDuma to make sure they were correct per my ISP. I left UPnP ticked in all areas. Using the port table in my primary router as a guide I forwarded every port that the Duma had opened. I ended up opening 12 ports!
After that the NAT has remained open. I will eventually be removing the port forwarding rules as a test. If it works just as well I will leave it as is.
This was just an experiment, and it's been working so far.