Verizon 4G LTE Network Extender - Server Error 5A - "ERROR: Fail to get certificate from CMP server"

I have poor cellular service at my residence, so Verizon has sent me a 4G LTE network extender.  This plugs into my broadband connection via ethernet, and acts as a local antenna to provide better reception for cellphone service.

The Verizon extender is not working.  I have already plugged it directly into an ethernet port on my router, created a static DHCP assignment, and put it as my DMZ server on my router configuration.  Although the extender obtains an IP address and identifies a DNS server, the Verizon extender cannot establish a connection with the CMP server.  I continually get an error in the network extender web interface:

Get certificate form CMP server ...

ERROR: Fail to get certificate form CMP server

And on the display on the extender unit it says:

Server Error 5A - Restart device & wait 1 hour.  If error persists call 800-922-0204.

In desperation I plugged the extender directly into the LAN port on my cable modem, which failed with the same error.  

I have called both Verizon and Xfinity many times and spent hours on the phone with both.  Verizon tells me that their backend is configured correctly and directs me to have Xfinity remove upstream network restrictions that prevent outbound CMP connections to Verizon's servers. 

Xfinity says that they have no restrictions and says that the problem must be caused by my router.  However, it cannot be my router, as I have already (a) configured the extender as my DMZ server, and (b) confirmed that it does not work when I physically bypass the router and plug directly into my cable modem instead.  On my most recent attempt, Xfinity's rep (Michael) refused to transfer me to a supervisor or next tier of support, saying that the problem is either with Verizon's hardware or with my customer-owned modem or router.

So I am completely stuck, without an escalation path to someone knowledgeable at Xfinity to help me solve the issue.  Has anyone figured out a way through this?