Hello,
They are probably reporting that issue, since the Mac Pro (Early 2009) isn't officially supported past Mac OS X 10.11.6 El Capitan. It sounds like the "hack" to install Sierra or High Sierra doesn't provide the proper hardware driver for the Broadcom module. I am not a proponent of running versions of OS software beyond the officially supported versions, due to issues like this.
There are a couple of things you could do:
1. Install Sierra or High Sierra on an external hard drive and startup from it. You could then use an Ethernet cable instead of Wi-Fi for Internet connectivity to enable use of the latest TurboTax software.
2. Use the TurboTax Online version, which supports Macs running everything from Mac OS X 10.6.x Snow Leopard or later. It also can be used with an iPad (3rd Gen) running iOS 9 or later.
Hope that helps,
C