Lacking a built-in ANT+ radio, you could buy an ANT+ USB stick to add that capability, connected via USB-OTG adaptor or hub. Alternately, you could buy an HRM with both ANT+ and Bluetooth capability. The Polar H10 would be a good choice or the Garmin HRM-Pro+ if you need the extra features that offers.
You probably need to force close (from the home screen of your tablet touch the button that brings up all the apps that are running in the background and swipe the Wattbike app to close) the Wattbike app then open the Zwift app and pair the HRM to Zwift. A lot of bluetooth devices can only connect to one app at a time.