This is just based on what I had done and was able to do, and your solution may be different.

TLDR: Make a Hoyolab account on a PC, link your game, and log into your mobile device's EMAIL login option. Do not use the mobile device's Google login option, it DOES NOT recognize it how it should.

DETAILS

I had found out about this app from a friend that was getting back into the game for Fontaine. I like that we can just take a quick look on our account without having to open the actual game, especially since I work near 11hr days and breaks are just not long enough to be waiting for the game to load.

Now I had created a Hoyolab account on the phone app first using the Google login option and in trying to link my Genshin account, it seemed like nothing was happening. It kept telling me something to the effect of "character data could not be found" but nothing was letting just link my account and get it over with. I remember getting a few error messages saying the account exists, even when I still had yet to link it, so I had no clue what it was referring to.

I ended up making an account on my pc and linked my Genshin account with no issue, but it still stood that I couldn't link it on the phone's account. Logging in still took me to the account saying it "couldn't find character data". I figured maybe I should do something different with the log in step. So I decided to choose the Email login option instead of Google. AND IT WORKS! Finally I can check my account through my phone.

Hopefully this helps someone in any way. Have fun!