Okay, I've watching threads about this for years now. 

I've been locked out my Archer c8 ac1750 for YEARS, nothing I did worked.

No username input, only password input for login screen on WebUI. Default password "admin" never worked. Nor any other default password suggested.

Tried resetting to factory settings, nothing.

Tried 30-30-30 rule, nothing.

Trie using WPS button instead of reset button, nothing.

Tried 30-30-30 using WPS button, nothing. 

Tried using theter app, nothing.

I never changed the default password (admin).

Followed many threads like this, this and this. May I say, TP-link closing the threads with no conclusive responses did not help at all.

After giving up, I decided to reset the router firmware via TFTP.

After trying latest firmware, it did not work.

Although, I tried the first one released just after.

It did not work BUT, after I gave up and disconnected the router from the power supply for some minutes, I tried just one more time.

After reseting using the first released firmware, I released the reset button way before the 10 seconds (because I was really annoyed already) and BOOM. 

It asked for the new password after accessing the webUI.

I don't know why. I tried this SO MANY TIMES. This router is really expensive where I live so I wanted it to work, but with no avail.

I don't really know if it was the firmware update, the disconnection from the power supply or the early release of the reset button. Also don't know if beign connected to wi-fi this last time was the solution, and I don't really wanna to try and find out anymore.

But since I found out MANY posts from users with no help other than the FAQ responses from tp-link, which was no use, I decided to post my solution.

I hope this reaches someone who is going through the same stuff. Cheers, and happy new year (btw I spent my NYE trying to fix this, tks to tp-link)