Hi,

For advertising I don't use the hcitool cmd, however I think a fast and easy check can also be done by using either the bluetootctl or the btmgmt tool.

So after executing the bluetoothctl you can activate advertising by just typing : advertise on. I would also execute : discoverable on,  otherwise the nRFConnect will not see the BT device that is advertising.

For a more user friendly approach I would recommend to use a C++ implementation (See e.g. QT heart-rate server applic) to approach the Bluez stack.

Best regards,
Frank