At this time, the Android App Support does not support Bluetooth connections to external smart devices. The only functionality currently available is Bluetooth audio connection and remote-controll-features to headphones inside Android App Support. However, I have heard that Jolla is looking for a way to make it work, as it will become much more important in the future.
What is currently working with the native SailfishOS app are some Amazfit smartwatches (Huami). You can get the latest SailfishOS-App only here at chum-repository.
From the Amazfish-Readme:
## Supported Devices
There are 3 tiers of supported devices:
### Gold
These are devices I have, have tested and will try not to break any functioanlity for an include:
* Amazfit Bip
* Amazfit GTS
* Pinetime Infinitme
* Bangle.js
### Silver
These are devices which are properly implemented in the application, but I do not have and are tested by the commuity, and include:
* Amazfit BipS
* Amazfit Bip Lite
I do not promise not to break funcaionality in these, but will fix where I can
### Bronze
These are devices which use a protocol that is close to another suppored device, and so is treated like that device. Your mileage may vary with these devices.
* MiBand 2/3/4
* AMzfit Cor
* Amazfit GTR
More instructions about that on the Openrepos.net-Webpage