Simply having the SentinelX nearby while mining on COIN will alert the Sentinel to stay awake.
If you have any difficulty with this, please try restarting your phone and make sure both the COIN app and your phone's system are up to date.
Once this is done, press the button on the SentinelX and open up COIN. You should be all set to be connected, but repeat pressing the button if necessary as COIN will tell it to stay awake.
Please be aware that if your phone is connected to another Bluetooth device, this other device will hold priority in your phone and you may experience in and out connectivity.
If you still experience connectivity issues after this, please refer to the article below:
All mobile software – apps – have some minimum system requirements, meaning that the version of either iOS (for Apple devices like iPhone, iPad, and iPod Touch) or Android (for Google-licensed devices like Samsung, LG, Motorola, Nokia, and Google itself). These operating system requirements are needed to make sure the app can run all of its necessary features.
You'll find a full explanation of how to find your system information and what the requirements for the app are here.