I finally got the Trailer Assist to work.
I am going to list all of the coding below, so that someone in the future will have it all in one place. Please note that I am using the OBDeleven Next Gen Pro on a 2020 Q8 Prestige with rear wheel steering. The coding may differ on other years or models.
Please note that the Trailer Assist icon will not "light up" on your MMI unless you have the vehicle attached to a trailer, the wires plugged in, all of your trailer lights are functioning properly, and your rear liftgate is closed.
Please also note that in order for the Trailer Assist to work, you must also have the Maneuvering Assist coded properly. It does not matter which one you code first. Here are the steps:
Maneuvering Assist:
09 Central Electrics > adaptation > maneuvering assistant > activated
44 Steering Assistance > long coding > byte 2, bit 1 (active)
A5 Driver Assistance > long coding > byte 44 > bit 5 (MA Steering Active)
A5 Driver Assistance > long coding > byte 44 > bit 6 (MA Braking Active)
A5 Driver Assistance > long coding > byte 44 > bit 7 (MA Maneuver assist installed)
19 Gateway > adaptation > installation list > parking assistance > coded
Important note: Make sure to code and swipe the installation button at the same time for the following two steps since they apply to left and right brakes and must be coded the same:
03 Brakes > long coding > byte 10 > bit 0 > active
03 Brakes > long coding > byte 24 > bit 7 > active
Trailer Assist:
44 Steering Assistance > long coding > byte 1, bit 2 (trailer maneuver assist active)
46 Comfort System > long coding > byte 4 > bit 7 (trailer monitoring)
09 Central Electrics > long coding > byte 28 > bit 7 (ARA installed)
A5 Driver Assistance > long coding > byte 15, bit 4 (FW ARA BCM installed)
A5 Driver Assistance > long coding > byte 54 > bit 6 (TMA trailer maneuver assist installed)
A5 Driver Assistance > long coding > byte 54 > bit 7 (TMA trailer maneuver assist image processing installed)
5F Multimedia > Adaptation > Car Function Adaptations Gen 2 > menu display trailer assistant > activated
5F Multimedia > Adaptation > Car Function Adaptations Gen 2 > menu display trailer assistant over threshold high > activated
5F Multimedia > Adaptation > Car Function List BAP Gen 2 > trailer assistant 0x36 > activated
5F Multimedia > Adaptation > Car Function List BAP Gen 2 > trailer assistant msg bus 0x36 > terminal 15
IF YOU HAVE REAR WHEEL STEERING YOU NEED TO PERFORM THE FOLLOWING ADDITIONAL STEP:
09 Central Electrics > long coding > rear axle steering > Hal not available
Regarding the rear wheel steering setting, I intend to turn this to "not available" when trailering and to "available" when not trailering.
I hope this helps someone in the future. Thank you everyone for your patience.