I think there’s quite a bit of discussion about Garmin watches (Garmin Fenix specifically) here on the Forums, so you can search and find more information about that.
Like Gerrie said, make sure the Fenix 5 can broadcast via BLE, otherwise ANT+ is likely your best bet.
From what I’ve seen, there are some Zwifters who claim that other Garmin watches work great in Zwift for heart rate when paired via BLE, but some riders seem to have problems pairing the watch using BLE. It’s hit and miss. I haven’t seen as many issues in general when trying to pair Garmin watches to Zwift using ANT+ and it seems they’re more directly compatible when using that pairing method as opposed to BLE.
If you want to stick with Android instead of using PC, there may be a way to use ANT+ on that. For more information, see this article.