ORBI AX4200 to ISP Fiber optic modem or router

Yes if the tech connected the RBR to the ISP provided router (ZYXEL XMG3512), this is double NAT. One of the options mentioned will help with this. 

FURRYe38 has already listed your main options.

But your list of devices is puzzling to me at least. Take this:

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(2) the ISP provided router (ZYXEL XMG3512). I mistakenly called this a modem in the original post.

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I suspect that FURRYe38 did the same as I did – ask Google about the ZYXEL XMG3512. The link I found describes this as a " VDSL2 Bonding/SFP Multi WAN Gateway". If so it is a  modem. The gateway word is sometimes used interchangeably with modem, although some prefer to reserve the gateway word for a modem that also support a telephone.

This then leaves me puzzled about this bit:

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The ISP supplied 3 pieces of equipment: (1) The fiber optic modem (Alcatel-Lucent 3FE51880AK I-240G-C )

 

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Google is less helpful there. So your network is a mystery. What does this fiber optic modem do?

If the Alcatel-Lucent is a modem, then you have two modems strung together. Optical followed by bonded DSL. Which is kind of strange and not something I have ever seen talked about here.

You also said earlier:

He also said the ISP router is required since everything is running through it - including my HD TV and internet.

 

To me this does not suggest that you must use this ISP router, simply that he set thing up like that. Done properly, you might not need it but you'll have to configure everything carefully.

Then again, we are left with this puzzling chain of modems.

There's another small fly in the ointment. Your first message said you are trying to configure an ORBI AX4200. That could be one of a few things. AX4200 tells us about the wifi technology, not the model number of the device. That is probably the RBK752 or the RBK753.

In an ideal world, you would ditch the ZYXEL XMG3512 and connect your ORBI to the Alcatel-Lucent "modem". The plug everything into the LAN ports on the Orbi devices.

FURRYe38 is better than I am at unravelling these conundrums. So they may have more useful suggestions, but I would certainly like to now what this Alcatel-Lucent is doing.

It  might help to say what sort of Internet do you have? DSL? (That is what the ZYXEL XMG3512 works with.) Cable? Or optical fibre? (Fibre alone is a meaningless label that ISPs use to describe just about ever type of Internet because they all have fibre somewhere in the chain.)

The ZYXEL HLA3000 seems to be something that you can plug into any Ethernet socket with an Internet connection.

Now, perhaps, you can see why people did not rush to answer your first message.