Question What card can you recommend for this NGFF M.2 slot?

It is likely going to be far easier to use a USB type of cell modem that supports 5G than trying to find a internal card. You also will need antenna of some kind for a internal solution.

The big problem with any mobile broadband recommendation is it all depends on which company you are using to provide the cell service. The end device itself must have radio support for the frequencies that the provider has licensed on the towers. It can be different in different cities in the same country and is very likely there are units that only work in certain countries.

Even beyond the technical messiness there are some brands the provider will not activate on there network. For example huawei stuff is not allowed by some providers in the USA.

Best source of this type of information is to contact the ISP you plan to use. They will have equipment recommendations but you might have to call to find a internal cell m.2 cell modules that they say will work on their network.