This is an intentional limitation to reduce cellular broadband bandwidth usage. If you require more than 5 minutes of Live View, we recommend adding your device via IP Domain on the Hik-Connect app with port forwarding. 

Source: https://supportusa.hikvision.com/support/solutions/articles/17000128663-why-do-my-cameras-time-out-after-5-minutes-when-using-hik-connect-