Your Exp-E node needs to be capable of reaching your Exp-C node.
The Exp-C node will have a Private internal IP address on the LAN so the Exp-E needs some way of routing traffic to this.
Does your Exp-E node have a single NIC which uses a Public IP address?
There are two main ways of setting this up but it depends on your setup.
Look into the 2 NIC setup for the Exp-E box, one NIC has the WAN Public IP and is reachable from the Internet and the other has a LAN address (can still be in the DMZ). You then punch ports in the FW between the LAN address of the Exp-E box and the LAN address of the Exp-C box.
A traversal zone is needed between Exp-C and Exp-E so giving the Exp-E box a Public IP with no ability to route back towards the LAN is not going to work.
The documentation on this is very comprehensive, the only difficult part in my experience is the Certificates!
The SRV record you are referring to allows Jabber clients which are connected to the internet to reach the Exp-E's Public IP. You also need an SRV on the internal DNS to allow Jabber clients to reach the internal CUCM/CUC/IMP servers whilst connected to the LAN, you don't want them going out to the internet and back in via Expressway if they are connected to the LAN anyway.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.