You can set a password in Handyman Office, so that the mobile user cannot change the IP address for synchronisation without entering this password on the mobile device. (Handyman Office -> System -> Settings -> Synchronisation)
To see what the password is you click the lock sign and the password will appear
On the device it will look like this when the server address is locked:
When you click the lock you will be able to write the password to unlock:
How synchronisation security works?
For a full explanation of how Handyman Mobile authenticates devices, encrypts data in transit, and the different network setup options (direct, DMZ, VPN), see How synchronisation and security work in Handyman Mobile.