How can I purchase such a protable oxygen concentrator if I'm not in the U.S?

Hi all,

My father has got COPD for nearly 6 years, I want to buy him a Inogen G4 oxygen concentrator online, since it it not available now in China.

However, it seems much more complicated than I thought:

1. I need a prescription from a certificated doctor in the U.S;

2. My father cannot travel that long to meet a doctor in the U.S, and the COVID-19 makes it harder;

3. I've asked some "online doctors" for medicine refilling or something similar, all of them said it's not possible to make a new prescription to my father via Video chat, even with a translator with him.

Anyone can help me work this out? I have all the documents proving my father's condition, I can make translation, certification.