Well, I had the same problem yesterday! I looked it up and the problem has to do with net NAT Type your internet connection has. If you do the connectivity check on your Switch, you will see the NAT Type. Mine was D (or even F) yesterday and to visit others in AC:NH, you will need NAT Type A or B.
I used this video and some information on the forums of my internet provider (which sadly is in Dutch): https://www.youtube.com/watch?v=WZ_icMBGIcg. Let me know if this worked! Otherwise, I'll have to translate a bit.