Xamarin Mobile App cannot connect to on-premises Web API via IIS

We have a Xamarin Mobile App that coonects to a Web API. The WEB API is hosted on prem using IIS.

We can use Postman to obtain a Token and then call one of the Endpoints i.e. retrieve a list of users.

However when we try using the Mobile App with the same Endpoint it will not connect to the Web Service.

They are using a VPN to connect to their Local Network so should have permission but we think the Firewall is blocking the VPN traffic.

We have been asked if there is a port no that is used by the mobile phone. We have asked for 80, 443 to be added but still no luck.

The Mobile Phone can open up a browser and navigate to the Endpoint but just doesnt work via the Mobile APP.