@do0zer 

you might want to read through this thread https://community.tp-link.com/en/business/forum/topic/622852?sortDir=ASC&page=1

in particular this

Upgrading the old version of firmware to the latest version prompts a username and password error because the new version fixes a replay attack vulnerability and updates the verification mechanism during login (adding a timestamp, which is equivalent to a change in the encrypted content of each login password). However, due to the browser cache, the password will still be verified using the old encryption mode before the upgrade, resulting in login failure. Clearing the browser cache and relaunching your browser can fix this problem.

  @MisterW 

I have tried to login with completely different computers: and the login still does not work.

  @do0zer 

in that case it seems you have a different problem. You need one of the TP Link staff to assist...

  @do0zer The port forwarding not working means you cannot successfully ping the WAN port or you can connect to the LAN side but can't login?

Have you ever tried cleaning the cache before manually logging in with your username and password? Whichever computer you use.