Honeywell wifi thermostats have a bit of a reputation when it comes to playing nice with even the simplest home network. It only gets worse on slightly more sophisticated environments. But I wanted to put all my IOT devices on their own VLAN for security, so it was time to face the angry bear.
Note that many hours were wasted due to my own ignorance, and a remarkable lack of practical examples in product documentation. Sometimes I thought that the manufacturers really didn't want anyone to succeed at this. Having said that, it's clear from the many responses to questions in customer forums that isn't actually the case. It's just the operation of the old adage, "Those who can, can't teach".