Headphones not connecting automatically

Hi there! I would first make sure that no other devices are connected to your headphones as some previously paired devices have the tendency to try to connect.

I would also reset the headphones. If they are the sport earphones, press the Volume Up And Volume Down for 3-4 seconds until you hear a beep, then try to connect the device to your iPhone again. You can see how to remove/add paired devices from your iPhone below

Pair a third-party Bluetooth accessory with your iPhone, iPad, or iPod touch - Apple Support

I think most people can't answer that simply because it's not an Apple's AirPods soooo. My best suggest to you is simply hold the side button and the volume up or down at the same time. Restart it, then remove the device and reconnect them! Hope this helps :)