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.

@zimplicitei I have a PC that runs games several games on the Epic Games platform and others, and they run fine. While researching your complaint, I came across a few posts that suggest the NAT type will impact the connectivity, as well as creating a separate 2.4Ghz or 5Ghz SSID specifically for your Nintendo gaming. 

https://en-americas-support.nintendo.com/app/answers/detail/a_id/12472/~/compatibility-between-nat-types#:~:text=A%20user%27s%20NAT%20(Network%20Address,Open%2C%20Moderate%2C%20and%20Strict.

https://es-americas-support.nintendo.com/app/answers/detail/a_id/22541/~/unable-to-connect-with-others-online-%28errors-during-match-making-process%29#:~:text=NAT%20Type%20D%20indicates%20that,improve%20your%20network%27s%20NAT%20type.

https://www.reddit.com/r/tmobileisp/comments/ruaxau/nintendo_switch_nat_issues_with_tmobile_gateway/

Let us know how it goes. 

Thanks! We've tried all that. It seems to be a connection issue between T-Mobile to T-Mobile.

Don't forgot to update your router's firmware and some will do it once connected to the Internet. With every update you'll need to do a "factory reset" as I got a new TP-Link router and got no connection issue again. After that reset and same set-ups it connected to Worldwide VS race on first try again.

Stuff not working above, please read: turn off all features for 2.4/5.0 GHz to get your T Mobile router/modem into bridge mode so it will not have double NAT if you also have a "router" connected. LAN is your connected devices and WLAN is your wireless networks which you'll need it at: 2.4 GHz or 5 GHz WLAN connection disabled. My TP-Link finally said, you got double NAT issues, so try to turn on bridge mode and reconnect all wifi devices to your TP-Link device. Connected to Worldwide - Battle on first try.

From what I've read about the features exposed on the TMo modems, the ability to enable bridge-mode is disabled/obscured.  The next best-bet is to enable a DMZ on the same router, and put your secondary router in the DMZ.  

Regardless, you're behind a CG-NAT (Carrier Grade NAT), so most IPv4 (port) forwards are going to fail given that you're sharing public IPs with other customers (this is sort of the definition of CG-NAT, or at least part of it).  The only (maybe) workaround is to use IPv6 explicit forwarding, assuming your router supports this, and you have the requisite experience to set these up (significantly trickier than v4 forwards, even with core v6 knowledge, IMO/IME), and knowledge of the v6 targets on the other side…

If you want to have a persistent public IP, the business service is the only option I’m aware of, where you’ll get a static assignment of an address or pool of addresses (depending on plan).