Mobile HDR is so easy to use. I downloaded the 3 bracketed pics direct from DJI Go 4 then save to camera roll. Open the Mobile HDR app select the 3 photos and customise the desired filter and save. All completed on the fly with iPhone X.
link to app in Australian App Store.
Mobile HDR by Intellsys s.r.l.
I’m not affiliated with or paid to promote this app. I’m just recommending this as a solution as I found it so good.