App linking to external site: Google log in not working on external site

My app links to an external site where the user can fill in some information before verifying that information by their FB or Google accounts (choice). The Facebook log in is working, the Google log in isn’t.

If the external site is accessed independent of the app the log in works no problem. So it appears the problem exists only via 3rd party app.

I contacted the external site in question who said that each customer needed to contact them individually. THe point of my app is to make life easier not more difficult.

Any ideas on how to go about this? sorry if it sounds vague, I’m trying to be brief and to the point.Thanks!!