Opening a port / port fowarding

my nest thermostats recently cut off and the nest tech support said i needed to open a port on my router for devices to prefent them from being shut off. 

I have been watching you tube videos and scanning forums but have yet to find a comprehensive way of accessing these devices in my router and moving them to an open port. 

I can find the IP address that the nest currently has from my router and go to the port forwarding screen in the setup of the router, but I dont understand how I tell the router to port that device specifically or open the port for the device with the Ip address it has assigned. 

when i get to the add custom service tab there are a lot of values that need to be entered and Im at a loss of the values that need to be entered.  

So I have devices that need to have an open port and unrestricted access.  Is there an easy to digest tutorial or instruction on how to do this? 

Thank you in advance.