Found out how to apply the barrier film!
After zero help from a Dexcom support person tonight (transferred me to Tandem) I found the answer to how to apply the barrier film under a Dexcom patch: you punch a hole in the barrier film, then remove the sensor patch paper with the code number and apply the sensor patch to the back of the barrier. Then remove the paper protecting the barrier film’s sticky side and put it on and inject the wire. That way you can line up the holes exactly. A true genius posted a YouTube video of it back in 2018. https://youtu.be/Mtl6KN1PZtU
When I looked at my first sensor that failed after 3 days I found that by putting the barrier film (hydrocolloidal bandage) on my skin first I had missed the hole in the bandage and injected the sensor wire through the hydro gel beside it.
The YouTube video also recommends a barrier film that is just a one-layer tape, not the gel-filled hydrocolloidal bandage. Smith & Nephew 5466000040 Opsite Flexifix 2 Inch x 11 Yards Transparent Film Dressing. Available on Amazon https://www.amazon.com/gp/product/B0070P81CW/ref=ppx_od_dt_b_asin_title_s00?ie=UTF8&psc=1
Tegaderm did not work for me, since it is not vapor-proof. After a few days the rash developed.
Dexcom is still just flailing around on this one. But a T1’s mom figured it out two years ago.
Later note: the Opsite Flexifix did not work for me as a barrier film, and the rash developed anyway. I am back to 3M hydrocolloidal bandages. The gel may ooze our and screw up the sensor, but at least I don’t get the intolerable itching and rash.