If you still are in possession of your old phone with 2FA-app, you should logon using that phone first. Then go to My Account and select 'Reconfigure 2FA'. Reconfigure 2FA with your new phone and you are all set!
If your previous smartphone is broken, lost or any other way not functional, you have to request a 2FA-reset. During the logon-procedure, on the screen where Topicus KeyHub asks for your 2FA-code, press the link to reset your 2FA-code. After a Topicus KeyHub administrator approves your request, you will receive an email explaining what to do next.