Fix Nintendo Switch Splatoon Online error code?

I got Splatoon 2 for the Nintendo Switch. Unfortunately, I can't play online because the error code "2618-0516" keeps popping up. I looked up a lot of sites, but found nothing that works. I use a Fr! TzBox 7490. My current Nat type on the switch is D. Can someone help me please? I don't get any further.

Your NAT type doesn't match. For this you would have to go to the settings of your router.

restart the switch and your router. Maybe that's enough (?)

What exactly should I look for in my router settings to change the nat type?

Which router do you have?

https://www.nintendo.de/...98000.html

look at this

@ iGhost007 as described above I have a Fritz! Box 7490