According to the German site Deutschlandfunk the movie was filmed in the Hebei province.
“Der Film ist in Hebei gedreht, in der Nachbarprovinz von Peking.”
In English that translates as:
“The film is shot in Hebei, in the neighbouring province of Beijing.”
The movie is also mentioned in the Wikipedia article for for Jingxing County:
“The 2018 film ...