-First let me tell you this, i have a huawei Phone and  it doesn't support Google. I have petal search in mine but i managed to download beta chrome. So i don't have play store but as much as i remember there is the app ao3 on playstore.
I will advice you to use website more than the app tho. The app sometimes doesn't work its more better for writer to use it. Like easier to post stuff.

1• Google Ao3 click on it then it will look like this and you have to click on Log in.

2•Then you have to look down and click on get an invitation.

3•Don't Ignore and read carefully then you have to add an email then click on add me to the list

4• After that it shows like this, it gives you a date around which your account will be created and once the account is created you will get an email (the same one who added) the then voila. 👌