The main thrust of all those instructions on OBi110 with an IP PBX is to use the FXO port on the 110 to enable the PBX to make and receive calls over a POTS line. That is irrelevant to your needs.
Can you reach the OBi echo test and hear "Welcome to Obitalk ..." when you dial **9 222 222 222 from an analogue phone plugged in to one of the phone ports of the 504? If not, the 504 cannot reach the internet and you need first to sort out that network problem.
If the Zultys is using standard SIP you should have as minimum 3 pieces of info. The SIP proxy, the AuthID and the AuthPassword. Typically those are the only 3 entries that you need to configure on an OBi to get it to register.
How are you configuring the 504? Do you know and understand the difference between:
1. Using the 504's on-board web server.
2. Using the Dashboard at
obitalk.com.
3. Using Expert Mode at
obitalk.com.
If this is an environment with firewalls or proxy servers between the internal LAN the 504 is on and the WAN?