ACNH a friend can't visit me?

I my friends have been playing ACNH on the Nintendo Switch lately and also want to visit us often via online mode. Only it has not worked lately since I have a new router and Internet provider.

The strange thing about the story is that only a friend can't visit me. (Let's call him friend A) Everyone else with whom I've tried it so far could visit me without any problems and so could I. The joke is I can even visit friend A but he doesn't stop me. We heard it could be the NAT type and that's why we looked. I had type B and after a router restart I still had type B. Friend A had type D and after a router restart I had type A.

Before and after it didn't work and even on the internet I find relatively few solutions.

In the attached pictures you can see the message that friend A gets when he wants to visit my island

I would be very happy about your help

The nintendo server stop

For sure? Because I only have the problem with one person and only if that person wants to visit me.

Check out your NAT types in the connection test

https://www.nintendo.co.uk/Support/Nintendo-Switch/Unable-to-Connect-With-Others-Online-Errors-During-Matchmaking-Process--1512808.html

As I said above, one of us has NAT type A and another B. How do you change that?