Sonos’ software has never had the ability to enter in a password for a ‘public’ WiFi signal. You can search for ‘hotel’ for examples as to how to get around this. 
 

This won’t work. Public wifi prevents devices on the network to connect to each other. Sonos needs this - your phone is the remote for your Amp.

You could use a travel router to connect to the wifi network on the one hand and to your phone and Amp on the other.