So, you’re going to run into some difficulties because a lot of what you’re dealing with is, I think, specific to casaOS, which makes it harder to know what’s actually happening.
The way you’ve phrased the question makes it seem like you’re following a more conventional path.
It sounds like maybe you’ve configured your public traffic to route to the nginx proxy manager interface instead of to nginx itself.
Instead of having your router send traffic on 80/443 to 81, try having it send the traffic to 80/443, which should be being listened to by nginx.
Systems that promise to manage everything for you are great for getting started fast, but they have the unfortunate side effect of making it so you don’t actually know what it’s doing, or what you have running to manage everything. It can make asking for help a lot harder.