HEROKU ImageMagick not detected

Question

I'm working on MoviePy to burn subtitles into video. Using TextClip

        subData = processSrtFormatData(request.json['subData'])
        subtitles = SubtitlesClip(subData, styleForText)

        def styleForText(txt):
            return TextClip(txt, font='Arial', fontsize=12, color='white')

and when deploy to Heroku. It throws error like in image below

IMG_URL: https://i.ibb.co/0DLMvtc/Untitled.png

I tried to use different buildpacks on Heroku, include:

https://github.com/DuckyTeam/heroku-buildpack-imagemagick

https://github.com/ello/heroku-buildpack-imagemagick

But only https://github.com/ello/heroku-buildpack-imagemagick works.

The problem is the 'ello' use imagemagick 6.9 and failed to work with unicode (as shown here). However on local run with imagemagick 7.1, it worked perfectly.

My question is how can I set the buildpack 7.1 on heroku or is there any other way to work with my MoviePy TextClip on unicode

IMG_URL: https://i.ibb.co/xHNmGcz/Untitledsd.png

Many thanks! (sorry for the images, dont have 10 reputation to include inside the question)