Internet problems Nintendo switch?

I need help and a few tips on my Nintendo switch. As far as the Internet is concerned, I currently still operate it with Wi-Fi. I have tried several times to play mario kart 8 Deluxe and splatoon 2 comes up every time the error code 2618-0516 have already searched through google and all write change the ip address I had tried everything without any success.

I thank you for your help

You have to activate UPnP in the router and enable the switch to enable automatic port sharing or assign the ports individually for the switch.

How do I switch the automatic port forwarding on the switch?

Is different depending on the router. I have a FritzBox. If you have one too, follow the instructions here:

https://at.avm.de/service/supportanfrage/produktauswahl/knowlegdebasesupport/detail/de_FRITZ-Box-7560-A-CH/894_Automatische-Portfreigaben-einrichten/

Unfortunately, I do not have a Fritzbox. I'm a Huawei 5g router from Telekom. In between, there's a Devolo magic 1 mini distributor so that I can get internet access

Then you need to browse the router's settings. It should still have the UPnP function. And any router can assign port forwarding.