Huawei Ascend G525 — Huawei Nova Plus: Problem logging in with Google Account after Factory reset

Hi all,
Since I always used the fingerprint for several months to "log" into my phone, not rebooting my phone for months, I had a problem to remember my pin after a reboot. So the phone did not let me in, I think it was the right pin (device PIN, not SIM pin), but I am not fully sure.
The Huawei support told me to do a factory reset. I did, and also changed the passwort of my google account at that time. After the factory reset, I could not log into to phone with my google account. I used the same account as before, but of course with the new passwort. Via normal computer brower google login works perfect.
So I started researching and found out that there is something like Device Protection and FRP. So I waited 72 hours, but still could not log in. Huawei and Google support do not have a solution. Both thing the other one is responsible.
So what should I do to get into my phone again. Invoice etc. is here, I bought it 3 months ago, I can prove that. But it seems security makes it impossible for me to use it.
Any ideas?
Thanks Aron

Thanks a lot for answering, but I did a factory reset and have no access to my mobile web browser before authenticating with my Google Account and setting up the phone again (which fails due to the problem mentioned).

As for the second option: How can I access the Android Device Manager in my Google Account?

Thanks. Unfortunately, there is no device shown... in the device manager. Still not working to log in.