Back to my Mac, NAT, CTU setup, Airport Utility

I've used Back to my Mac from home to access my iMac at the office in the past, but after the Yosemite update, I am not seeing "Shared" listed in the SideBar of Finder Windows anymore, and I also get the following warning in System Preferences > iCloud > Back to My Mac:

Set up router for better performance.

Clicking "Details" yields this message:

Back to My Mac may be slow because NAT Port Mapping (NAT-PMP) or Universal Plug and Play (UPnP) is turned off on your router.

Let me explain my router setup

My iMac at home is connected to the internet via fiber optic line here in Japan. The fiber cable comes from the wall to a white plastic NTT box that has 4 green LED lights, the only one of which that blinks is labeled UNI. That box is the Ｂフレッツ ONU shown on this WIKI page:

http://ja.wikipedia.org/wiki/%E5%85%89%E5%9B%9E%E7%B7%9A%E7%B5%82%E7%AB%AF%E8%A3 %85%E7%BD%AE

The Fiber cable form the wall goes INTO that ONU, and then there's an Ethernet cable that goes OUT of it into the WAN port of another NTT box with a black front-face, Model 2004WS, which is called a "CTU" and which I assume is the actual "Router" in my setup. I then have an ethernet cable going between the LAN1 port of that CTU and the WAN port on my Airport Extreme Base Station (the WAN port on my Airport Extreme has what looks to be a sun-shine icon). Another Ethernet cable goes out of my CTU at the LAN4 port and leads into a dark blue VOIP Adapter box. And the LAN3 output of my CTU leads into an Ethernet Hub, from which I connect my iMac and SE/30 (yes, that vintage Mac can connect to the internet!). (My iPad connects wirelessly.)

The 3 NTT boxes I just described are shown in this photo:

http://hoe.moe-nifty.com/hoe/images/HI350155.JPG

If I open Airport Utility 6.3.4, click the Airport Extreme icon, then click the Edit button, within the Network tab, I see the "Router Mode" is Off (Bridge Mode).

When I launch Network Utility and click Traceroute and type "icloud.com" and click the Trace button, Line 2 shows three asterisks " * * * ". That seems to be saying that I DO NOT have a "Multiple NAT configuration," as per this:

http://support.apple.com/en-us/TS1208

I can log into my CTU via Safari using the following URL:

https://ctu.fletsnet.com/CtuC231/init.do

At left there is a button named "詳細設定" which means "Advanced Settings." Clicking that button shows me various settings, and I can see that this CTU is acting as the DHCP Router, and that UPnP is enabled. There is no mention of "NAT" at all, so I assume that NAT is OFF on this CTU.

The following Apple Support article is suggesting that to resolve Back to My Mac issues, I would need to change the "Router Mode" from "Off / Bridged" to "DHCP and NAT":

AirPort Utility 6.x: Set NAT options for your base station or AirPort Time Capsule

But since my CTU is already acting as the DHCP server, wouldn't changing the Airport Extreme Router Mode from Off/Bridge to DHCP wreak havoc on my setup?

What should I do?

Thanks.

P.S. I also have a Brother Laser printer MFC-7840W that connects wirelessly via the Airport Extreme, and in the Network tab of Airport Utility "Timed Access Control" is enabled and that printed (wireless client) is enabled. It took me a heck of a long time to figure out how to get that printer to work wirelessly, so I don't want to mess it up and no longer be able to print if I switch from Off/Bridged to DHCP and NAT.