Nintendo Online cannot play with other T-Mobile subscribers

You'll need to buy a separate router (any brand you like: netgear, motorola, cisco, et cetera), connect it to LAN1, then on that router's splash page (usually on device with IP address) log on as administrator. Create a "guest" Wi-Fi network which most allow you to create a password but for me I have a Motorola router. It allows me to create a password as well as turning on (by default) / off "internet connection only" feature all at 5 GHz but I did not change any values, got my Switch on that guest-WiFi and connected to Mario Kart 8: Worldwide VS Race on my first try. No need to go to battle first for me, hope it works for you and thanks for reading my post.