I was able to upload a hex file to Google Drive on my laptop and then download onto my Android phone. Then I was able to import it to the app on the phone. But what a faff. Even if you uninstall the app and go to the website, you caan’t log in and none of you projects is visible unless you do what I did.
I had uninstalled the App, so I used a web browser. I clicked on the upload icon.
To help you, I have just reinstalled the app. From the main menu, go to ‘Let’s code’, then ‘MakeCode editor’. Next to ‘My Projects’ is the upload icon. Tap that and then select ‘Import File’. I had to hold my phone in landscape position to be able to read the icons. Next you click ‘Choose File’ and find the hex file you downloaded. I did have trouble at this stage! I could browse to the file and select it, but then, nothing happened! But after a few repeats it worked. I used Googles Files app to find it, are you using a different file manager?