Just got my new P3 SE. My Oneplus 3T phone has a USB C connection not a micro USB. The plug for the USB C is too wide for the slot in the base of the mobile holder on the Phantom 3SE so I cannot connect my phone and have it secured on the mobile holder. How can I over come this?

By buying a phone or tablet that is on the DJI recommended list:

        iOS： iPhone 5s、iPhone SE、iPhone 6、iPhone 6 Plus、iPhone 6s、iPhone 6s Plus、 iPhone 7、 iPhone 7 Plus、iPad Air、iPad Air Wi-Fi + Cellular、iPad mini 2、iPad mini 2 Wi-Fi + Cellular、iPad Air 2、iPad Air 2 Wi-Fi + Cellular、iPad mini 3、iPad mini 3 Wi-Fi + Cellular、iPad mini 4 and iPad mini 4 Wi-Fi + Cellular
Android: Samsung tabs 705c、Samsung S6、Samsung S5、Samsung NOTE4、Samsung NOTE3、Google Nexus 9、Google Nexus 7 II、Ascend Mate7、Huawei P8 Max、Huawei Mate 8、Nubia Z7 mini、Sony Xperia Z3、MI 3、MI PAD、Smartisan T1

Phantom 3 SE is connecting the mobile device via WiFi, please go to the WiFi settings and find the WiFi that the RC launches, the default password is 12341234. If it doesn't work, it might be the compatible issue, then you might need to try another device. Below's the tutorial. And for the RC's holder, refer to the pic below.