IPad doesn’t have ant+ just Bluetooth so if it is the old Garmin HR then it will only do Ant+. It appears the S8 has ant+ so I would have expected it to work but according to the googles that is not the case (see section on Android) How to set up Zwift to try indoor training on any device | road.cc
But then later I see this earlier post about android support
In mentions you should press the ANT symbol top left in the pairing screen. It might be worth having a little read there might be some other steps.