You have the SBG8300? 

Have you tried setting the 2.4ghz wifi channel? try setting 1-6-11. 

Its usually set on auto and maybe the thermostat isn’t changing when it “auto” changes the wifi channel. 

I tried changing the Channel to manual and  the channel to 1-6-11

this is the current setting

Mode:  b/g/n

TX power: 100%

Channel selection :  Manual 

Channel:   6 

Channel bandwidth:   have tried 20 and 20/40 and Auto

The Carrier thermostat will stay connected for approximately 5 minutes and then disconnects. There are a multitude of combinations that I have tried with these settings. I could be getting the sequence wrong on 1 or more. Any clue as to what is causing the time interval of connecting for 5 minutes and then disconnecting ?

If you do a quick check on carrier thermostat disconnecting, there’s a bunch of posts about it on multiple sites. Some were fixed by downgrading the thermostats firmware.  Others were fixed by either changing the DNS server or using IPv6 (rebooting the thermostat after changing those. I’d do a quick google search on carrier thermostat disconnects and check to see what a few people have tried.  

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Carrier Thermostat Disconnections | Ubiquiti Community