Sign-In without changing DNS
To check if an account works without changing the DNS environment it is possible to modify the host file. With this it's possible to test internal usage, federation is not possible.
Solution
Go to "C:\Windows\System32\drivers\etc"
Modify the hosts file as local administrator.
Add two lines:
95.211.37.68 sip.<domain>.<tld>
95.211.37.66 lyncdiscover.<domain>.<tld>
Sign in on the client