Hi
I have an RB2011 router board with Ubiquiti access points running a captive hotspot. The Incoming Ethernet is via a leased line managed router, our interposing Draytek router and then onto the Mikrotik which is set to DMZ in the Draytek. We need to add a number of Grandstream WiFI phones which are meant to log onto a hosted IPBX. When a phone is connected on the WAN side of the Mikrotik it registers. When it is connected to the LAN side it doesn't. The SIP Helper is off. The MAC address of my test phone is whitelisted in the hotspot IP bindings and walled garden. The IPBX is whiltelisted (bypass) in the wall garden. From the WAN side I can ping the IPBX. From the LAN I can ping to bbc.co.uk, the Draytek and the site Public IP address but not the IPBX. We have used this hosted provider for years and never had an issue with firewalls as the initial registration is outbound. Here, if I cannot even ping the IPBX I have no chance of registering to it.

Can someone give me some sanity and help me navigate through this?