Here is how to configure Xabber Beta for Android in Nethserver NG 6.8 - maybe it works in NG 7 also..
I have Xabber Beta in italian so I will try to translate the steps in english...
Create a new account:
Tap on hamburger (the three orizontal lines icon on the top-left corner) -> select "XMPP Account" (Account XMPP)
click on the "+" icon in the top-right corner
Username (nome utente): firstname.lastname@example.org
Save Password (Memorizza password): checked
Other options: unchecked
Tap on "Add Account" (Aggiungi Account) on top-right corner
You will have a "error connection": don't worry!
In the new option list that you have visualized:
Personalized host (Host personalizzato): checked
Port (porta): 5222
Clcik on the arrow on the top-left corner
Voila': you have your account online
Maybe it will ask you for the certificate: accept the certificate