Michelle, we have the same system as you. Our WFR Pro was bricked when 6.6.3 first failed to install. It did take a couple of weeks to get a RMA replacement WFR Pro, which did not meet expectations from what was stated in their initial general responses in the forum. But I also understand that there was an instant load of issues to handle with limited resources/time.
The Boost firmware can be upgraded but it's a Ubiquiti firmware. I have successfully updated it and I doubt this is supported by WFR. I understood this but was at the point I couldn't do anything else except buy a new Bullet or a whole new system. So, I jumped in knowing the potential consequences. The firmware was downloaded from the Ubiquiti Support site. You might want to be familiar with the wiki and the forums before attempting this upgrade because an unsuccessful firmware updates can render the device/bullet dead - e.g. 6.6.3 WFR Pro . All that said and done, the boost does work with the replaced WFR Pro with 6.6.3 but it is flaky. There is an instability in the system that randomly drops the Boost/Bullet and requires me to go through a set of hoops to get it connected back to the WFR Pro (Set up Tab|settings for boost| Select None|Save/Wait|Select Boost|Save/Wait|Sometimes reboot/wait). A key part of the process is to wait and the needed attitudes are patience and endurance. (On occasion these commodities appear to be in short supply, which is not true. Instead, this is a personal problem because I, the branch, am not staying connected to the Source, the Vine.) I have found the WFR to Boost/Bullet reconnection process through trial and error. There is also an internal instability that causes random reboots of the WFR Pro, which is not solved with 6.6.3. While Brian at WFR is very helpful, it is somewhat disappointing with the lack of documentation that WiFiRanger provides and the process of support. It's sad but this particular system (WFR Pro and Boost) is already old and to upgrade is a pinch in the pocket book for some (us).
6.6.3 will still support the boost running the UBNT firmware. There is no plan to deprecate that feature. However, there are some issues to be aware of. The most important, is that the 5.5.3 version of AirOS has a problem in the bootloader of the AirOS device, causing it to "hang" when starting. This of course causes the WFR to get erratic results from the Boost, and a domino of issues pertaining to that. If yours is one of those, that may indicate what is going on.
Check your firmware on the Boost, (via the CP WFRBoost link) or connecting directly. Make sure you "reset to Factory defaults" on the AirOS prior to trying to configure it on the WFR tab. Then re-configure.
Finally, we've seen with newer firmware on the Boost, that the power use has changed a bit. (Why boggles even me) That may cause the WFRPod power supplies to be stretched when both are running high power. Ask WFR for a AC POE device and they can get you one to replace the current one and that may clear up any power related re-boots that are happening.
2008 DynaMax Ultra Sport 45 (WFRMax)
450HP Mercedes, Allison 6 Auto, Cat GenSet
WiFiRanger GoG (775205), WFRMobile, WFRXtreme