No problem. The app would be able to connect to their internet services, it only do it connecting to your local guest network that is connected to internet but have not access to anything else on your local network using wifi, from a phone that has nothing else on it so so SIM, no contacts, no navigation cookies and so on. At least some other useless app like itself.
It is not the perfect solution (which would be to not need an app) but at least you neutralize any kind of spyware/data harvesting since there is no data to harvest if not the one generated by the app itself.