go buy an address book and keep it near your computer.

I have 100 passwords for different accounts at my fingertrips.

to a visitor 2 weeks ago. Think problem must be with the thermostat.

the manual. You should be able to download the manual. There is a reset button somewhere, instructions in the manual. After resetting it, you will want to go through the original setup instructions.

without controlling the thermostat via phone.

The original password should be printed there, on the router.

If you changed it, you still can get to it by going to the admin 'site' of the router.

It is a local site only available in your local network and it should be accessible through an address like this:

192.168.1.1

If it lets you in, you can go to the wireless section and look up or change your password.

The trick is whether you'll be able to access the site. I can, from my phone, because it remembers my password. If you changed it and your device does not remember it, I can only think of the previous suggestion: reset.

You type in the 192.168.1.1 in the address bar of your browser, the same way you type in any other address, like google.com

I would also try 192.168.0.1 and 192.168.0.100

If one of those does work, you probably have to log into the admin site. Try admin for the password.

*** But definitely check the back of your router, that would definitely be the easiest if it's there. I'm guessing you never changed it.

Silicon Valley ref

and it worked for them.
Must be the thermostat. I will contact their support about the timeout.