To further clarify, NAT type isn't a setting specifically, it's a status of how your network can be traversed externally (systems from the outside communicating with what's inside). Nintendo seem to have come up with arbitrary letters for theirs but the indicate levels of how well various
hole-punching methods systems work. A and B indicate types of Open NATs where perhaps
UPnP is used (or the port is straight up open) or the previously mentioned hole-punching. C and D indicate restricted NATs where hole punching cannot function properly. They have F as well which straight up means blocked, so a business or university network restricting UDP traffic, for example.
In your case, you are using a mobile network which is typically using what's known as a
CGNAT. These share a single WAN address per multiple nodes (you don't have your own IP, it's shared amongst other mobile users) so there's no port forwarding or UPnP method possible and it can only make outgoing connections. Hole punching may work in rare situations, but in most cases you're stuck only being able to communicate with Type A NATs as they can still accept incoming connections from you fine.
Note this won't affect every game. Quake for example uses Playfab services, so direct connections are never made but instead tunnelled through Azure (at least on consoles). Thus any type A through D will work just the same. Though never host a lobby on your mobile network, it will not go well.
