It depends on the airport. Some airports have exclusive deals with WiFi providers, so if you want WiFi in LAX you need to buy LAX's provider. A couple of the frequent flyer clubs use T-Mobile, so the odds are OK that you can get a signal outside of a United frequent flyer lounge even if you're not a member. The signal leaks outside the doors. But even that's not a given--the airport may prohibit the United club from using T-Mobile. Generally T-Mobile is a good bet, since that's also used by Starbucks, and outside the airport you can always find a Starbucks.

I subscribe to T Mobile.  Gets me all the major airports, especially inside the
AA Admirals Club, plus there is a Starbucks on every corner.

I can find a place nearly anywhere I go with them.

Some airports just have their own, as mentioned.  No tricks on those really.