1. Ensure that Mobile Hotspot is turned on
When you first enter the connections menu you will see Mobile Hotspot and Tethering. If you tap on this it will take you to another menu with Mobile Hotspot, with a button next to it to turn it on. Normally when you tap this button it will enable hotspot, however, there is another button you may need to turn on. From the same menu if you click on the word Mobile Hotspot you will be taken to a new menu with another an On button. Turn it on here to enable Hotspot.
2. Ensure Allowed Devices Only is switched off
As a security measure, Android phones have a setting which prevents devices from joining a mobile hotspot unless they are allowed. This will prevent the VeriFone vx690 from joining the hotspot network.
To turn it off navigate through settings > connections > mobile hotspot and tethering > mobile hotspot. In the top left corner tap the three vertical dots and then tap allowed devices. Ensure that allowed devices only is switched off before attempting to join the mobile hotspot.