I wouldn't put too much stock on the list of supported devices. For the record, I have never used a device that was on the list and it has always worked.
There is no way that a Nexus 6P should not work but stuff happens.
Perhaps forget the device and try to re-pair.
There are a number of threads and possible solutions on the Fenix 2 forum, some have helped, some not for everyone. Connectivity with F2 is difficult since version 3.10 of GCM and better but not great with 3.11 - installing an older version often helps, but not all older versions will work with a Vivoactive.