Here's the problem: High Sierra (as of 10.13) ships without the firmware update for this A/V dongle. This update package normally rests at "/System/Library/CoreServices/Firmware Updates", but this folder is empty following the update. So if you're stuck at an older Firmware Version (pre 2.33, article: Apple Support Article HT205858) there is no separate download for it, and you are on your own until Apple releases a hotfix.
If you know your way around a terminal follow this guide to get it solved. First off, make three checks; first that you are on Mac OS X High Sierra (Build 10.13 or greater), that your Multiport Adapter reports a firmware version below 2.33 (around 1.53 is old), and finally that the folder "/System/Library/CoreServices/Firmware Updates" is empty.
At a high-level, I guessed that I would find the firmware update package in an old Mac OS Combo Update file. Hunch turned out to be right, and I was able to install the firmware update. Lets trace those steps so you can do the same.
First up, kick off the download (around 1.5gb) of the macOS Sierra 10.12.6 Combo Update: https://support.apple.com/kb/DL1931
At the same time, ensure Homebrew & XCode utils are installed on your system from brew.sh
Install pbzx, for extraction of update pkg Payloads.
brew install xz && brew link xz git clone https://github.com/NiklasRosenstein/pbzx.git cd pbzx clang -llzma -lxar -I /usr/local/include pbzx.c -o pbzx #Install pbzx to path mv pbzx /usr/local/bin/
Once the download has completed, mount & extract the update dmg.
open macosupdcombo10.12.6.dmg #Expand pkg contents to tmp folder. pkgutil --expand /Volumes/macOS\ Sierra\ Update/macOSUpdCombo10.12.6.pkg /tmp/sierra/ cd /tmp/sierra #Extract payload contents to current directory pbzx -n macOSUpdCombo10.12.6.pkg/Payload| cpio -i
Move the firmware update package from the extracted files to a local path, we'll use the user's home directory.
mv System/Library/CoreServices/Firmware\ Updates/USB-C\ Multiport\ Adapter/adapterupdate1.pkg ~/
Finally, execute the installer and reboot. Before you execute the installer - ensure that the Multiport adapter is plugged in, with nothing else plugged into it. Ensure that you have power connected to one USB-C port, and the Multiport adapter to the other. Apple's article recommends doing USB-C power-pass through, this did not work for me. You can try different configurations if this doesn't work.
#Kick off the install sudo installer -pkg ~/adapterupdate1.pkg -target / #Then reboot sudo reboot
If all went well, System Information should report your USB-C Multiport Adapter is at firmware version 2.33.