"Could it be an issue with the fact that my phone is Chinese so cannot receive push notifications from the app?"
I doubt it as my OnePlus 8 is also Chinese and it used to work well until the last update to 20H2.
Actually that does not mean anything. The reason is that different Chinese phones from different manufacturers use separate Push notifications services because they don't access Google Messaging service. So Xiaomi has its own Push notifications service, same for Huawei, Oppo & Vivo etc. I am not sure about OnePlus which service they use.
So if the Phone Companion app on Android only supports push notifications using GMS (Google Messaging Service), it will not work inside China (therefore network already blocked by GFW) and also will not arrive on a phone that doesn't support GMS.
So that's why this distinction is important, and I think Microsoft should try to clarify. Inside China I've been trying many phones, most of them don't work, they can all sign in to Microsoft account successfully, but once you have to receive the push notification, none of them work.