VPN on mobile affects medical app Australia

I am in west Australia, I use a medical app for my health prescriptions. Whilst in my home, my phone connects to my house wifi Telstra, when I leave my home, I connect with my phone provider, belong. I Also have avast VPN active on my phone.

Whilst in my home I am unable to access my health app, but if I leave my home, I can access the app. I wrote to the app provider - who advised me that due to strict Australian rules, medical apps can’t work whilst VPN is active & I need to turn off the VPN whilst using the app

But I. Confused because if I leave my home, I can access the medical app? So does that mean my VPN has not registered whilst I am using my phone provider? Does the VPN only work whilst I am on my home wifi network?

This all seems very confusing? I am unsure whether I should continue with my avast Mobile protection… If I have this problem

Anh suggestions please :rose: cheers Heather