Airport Express and a ecobee3 lite

 I have not been able to get this to work and I am wondering if he seting causing the issue is the Router Mode: in my airport express. It is currently set at Off (Bridge Mode), I do not want to mess up my internet so am asking this question in order to prevent that.

Bridge Mode is the correct setting for the AirPort Express if all of your other devices are connecting normally.

In addition to the comments that you have already received from La Pastenague.........IF......you have a 2nd Generation AirPort Express (it has two Ethernet ports).........by default the Express will provide a wireless network signal that uses the same name for both the 2.4 GHz and 5 GHz networks. This is called simultaneous dual band operation.

Some devices.......your thermostat may be one of them.......have trouble connecting to a simultaneous dual band network. The solution in this case is to assign a separate name to the 5 GHz network on the AirPort Express and then "aim" or point your thermostat at the regular 2.4 GHz network.

This is confusing to a number of users, so if you have tried the suggestions that LaPastenague has already suggested and the thermostat is still not connecting to your network, post back for more instructions.

In that case, we need more information from you as follows:

1) The make and model number of your current modem or modem/router

2) What device you are using to reconfigure the AirPort Express......is this a Mac, a PC, or an iPhone / iPad?

Double NAT is a network error that should be avoided if possible. Ignoring the error will not make it go away.

As we said, if all the other devices are connecting OK in the Bridge Mode setting.....(which Apple calls "Off (Bridge Mode)"......than that is the correct setting for the AirPort.

We still need the make and model number of your "modem" or "modem/router" though to confirm. Or, if you are saying that you do not have a modem or modem/router......please confirm on that as well.

Your airport express is the old type.. with very low RF output for wifi. It is really unsuitable to use as a main router which is what double NAT will do.. and even in bridge is going to be fighting with all the wifi in your area.

I would seriously consider replacement with something not Apple (because Apple are now out of the router business).

Arris TG2472 is more than a modem......it is a modem/router, which is also known as a gateway. Bridge Mode would absolutely be the correct setting on the AirPort Express to work with a modem/router or gateway.

The AIrPort Express pictured in your screenshot is at least 6+ years old and as much as 10 depending on when it was placed into service. 4-5 years is about it as far as the average useful life of this product, so that may also be a factor here.

Bridge is more than likely the correct setting.

The issue could well be the wireless name (SSID) that apple recommends to users via the setup wizard.

The name might look like

Paul's Airport Express

If so change the name to something really short and simple with no spaces and pure alphanumerics.

eg PAE

Password also same rules.. but can be longer.. mixed case and numbers only.

Lots of IoT have issues with the airport routers and their strange Apple names and passwords.

No luck post back with all the details.

What modem router.. make and model?