You can find more information on how to setup drop-in mode here: Configuring Drop-in Mode
The instructions are a bit older, but should still apply, if they ported it back to the Balance 20 it’s a recent update as they didn’t in the past.
But just to clarify, the Balance 20 is meant for the second site only? On this location you have one connection coming from the main site and a second direct Comcast connection? Are you planning to use the Balance 20 as router there or just an aggregator for both connections (drop-in mode) with another router behind it?