This Xbox help article explains how having a "Moderate" or "Strict" NAT can affect you and how it can be solved:

https://support.xbox.com/en-GB/help/hardware-network/connect...

I don't think anything has fundamentally changed in this area since the days of the Xbox 360. Back then it was a PITA to get two or more consoles working properly without UPnP, and I can't see anything about the problem that would be different now.

Nintendo has a similar page:

https://www.nintendo.co.uk/Support/Nintendo-Switch/Troublesh...

So does Sony:

https://manuals.playstation.net/document/gb/ps4/settings/nw_...