Would you please download the videos to your mobile phone ? It will be good to view the videos ,same as Forsie's mentioned, the Wi-Fi is not too strong and the distance between the phone and the DVR will be a factor to affect the performance of the video.
Can you indicate how you view the video in the app? The more information you provide, the better we can help you. Keep Frostie's tip in mind.
I suppose you are trying to watch a recorded video in the app and did not download it before.
This means the app is streaming the video from the K3 via wifi. As the wifi of the K3 is not very strong and fast, the data rate may be too low for streaming, depending on the distance between the DVR and your phone or the mounting location of the DVR.
In general it is better to download a video before watching it.