On my iPhone (6.5.8), nothing will play. On my Apple TV, everything plays fine.
Are these files you have downloaded to your phone or are you streaming from a local server or from a cloud server?
If streaming from a local server what protocol? SMB FTP etc?
Are you able to see the files in the shares and library on the iOS device?
All these files are in a Synology NAS and being accessed over SMB. I can see all the files on ask devices running Infuse, even when I add more, all devices get the updated file listing. It stopped working recently, maybe after I inflated iOS to version 14.3.