Issues combining 2x mp4 h264 AAC video files - MUST play on Android and IOS

Hi Everyone, We have two video files: - [login to view URL] [login to view URL] (attached) We need to combine these two files together WITHOUT re-encoding, we CANNOT change the codecs or audio channels of the [login to view URL] we CAN change as much as required in [login to view URL] basically we have a video file [login to view URL] but we need to be able to insert [login to view URL] at the start WITHOUT re-encoding (it needs to be a quick process, we know we can re-encode). We already know how to combine these files with a piece of software or if you can tell us how to do it with FFmpeg that would be great. We want somebody to tell us: - - how to create an [login to view URL] video file that is compatible for joining to s4.mp4. (to a single mp4 file with [login to view URL] at the start and then [login to view URL] to follow it). Tell us what software and settings to use as we'll need to make different [login to view URL] files in the future. - the combined video must play on IOS and Android We can get the videos to join and play on andriod phones at the moment, on iPhones/IOS however the video doesn't play for the second part ([login to view URL]), the combined video file must be capable of playing on Andriod and iPhones/IOS. Our current process will not currently play on iPhones/IOS. We're guessing it's because the videos are different somehow (codecs?) We're tried various frame rates and audio channel work but we are stuck! Looking forward to somebody helping us! If somebody and send a sample mp4 file that is compatible with our [login to view URL] we can test it works. Thanks.